I would start with the basic oxygenators, like Anacharis or Hornwort, or floating aquatics like Water Hyacinths or Parrott's Feather. A clump doesn't cost more than a few bucks (or get it from a fellow pond person who has too much) and it spreads fairly rapidly if you have the right climate for it.

If you are looking for terrestrials that can survive in the pond then two I have had luck with are Impatiens and Hostas (I can't ID the exact variety of hosta for ya).

There are probably other cost effective pond plants at your disposal if you look around but you also didn't say where you were located. Just remember you might have cattails growing up in the wetlands around you but they probably haven't been cultivated and therefore can become invasive.

Look for some sedges, rushes and phalaris that people want to divide or give away. $25 sounds about right for a potted water lily from a water garden store. Another thing to keep in mind is that if you aren't careful you will end up spending more for the pots/containers than the plants. Try to find suitable alternatives to the $12 black plastic pots they have at the water garden store. Many on this board have used oil pans, used in draining automobile oil from an engine; these work well for the lilies because they shallow and round. One thing I've found around here, because I'm so close to farm country, is animal feed pans. Feed stores have them and they are black in color and relatively large and inexpensive.

Good Luck.

I've re-read your OP and I see that I may have been off-base with my response.
Not all irises will do well in ponds, if that's what you're asking. My terrestrial bearded irises like it mildly dry.

If you wanted a list of other terrestrial plants that have at least some record of performing well in bog or wet conditions, I have this list from a previous post (keep in mind, many go by different names, and, it certainly depends on climate --i.e.: I haven't been able to get watercress to grow but everyone else swears by it!):

Bee Balm
Bullrushes
Caladium
Callas
Cannas
Cardinal Flower
Chocolate Vine
Coleus
Colucasia
Creeping Jenny
Flax
Green & Gold
Hedychium
Hymerocallis
Impatiens
Pansy
Peace Lilies
Pothos
Rueilla
Schizosotylis
Snapdragons
Sweet Flag
Violas
Wild Violets
Impatiens Capensis

If there is a pond club or water gardening society in your area, you might think about joining. Many of them have "plant exchanges" or "plant giveaways", where members bring their excess and give them to fellow members. It's a great way to try different things at no cost at all.

Just wanted to add; I have read that 'Caesar's Brother' terrestrial iris does well in ponds as a bog plant. That is a small iris. I have had mine in a boggy area for a year now.
I have Blackberry Lilies next to my lotus pond, I let some seeds drop last year, and now I have some growing in the lotus pond. Aprox 5-6" of water. Also have some loosestrife in there I was experimenting with. It survived over winter and is doing well. But that can be invasive, so be careful. Mine is supposedly sterile, which after 4 years I think is true. I have never had any volunteers, and have only been able to get more by separating the root mass.
Also, there is Swamp Hibiscus, but it wouldnt be a perennial for you.
Bloody Dock, that would be an annual as well, unless you bring it indoors in the winter.
Dwarf cattails..

Many pond plants grow like weeds and there are usually people willing to share extras for postage or small fees on other water gardening forums. This would include waterlilies for a lot less than $25. I compost pond plants often even after selling some on eBay, trading and selling some on pond forums. Many are really aggressive growers.

Garden Web has an Aquatic Plant Trade Forum but it is for trade only, not for postage or sales. The link is at the top of this forum.

You may have garden plants someone would be willing to trade for pond plants.

There may be a local club or group as suggested already.

Can't help you with garden plants that grow in ponds as well, I like my garden plants in the garden and aquatic and bog plants in my ponds.

Wow, ask you and you shall receive :) There are some great ideas here. These suggestions have really helped alot.

I did post a "want" in the pond plant section and offered garden plants in exchange so I will see what that produces.

I used to belong to a local herb group so will check on to see if there is a local pond group. There is also a garden walk scheduled in town next week so may get some ideas there as well.

I didn't realize so many plants might do well in water. Thanks everyone.

Hi,
I can tell you from experience that the Flag Iris and Marsh Marigolds(caltha) are very hardy for your zone, also Arrowhead/Wapato(sagitaria). Pickerel weed is also supposed to be zone hardy but I only wintered it once and the pond was heated for the winter. I have had pretty good luck wintering non hardy plants in the pond with the help of a stock tank heater that keeps the water just above freezing. I also had good luck wintering plants in the crisper in the fridge. It makes it easier to pay the big bucks for the true water/bog plants if you know you can winter them and get them back the next year. The peace lily and the cardinal flower from Pondbucket's list, I have tried both repeatedly and they have never wintered for me even tho they are said to be zone hardy, my research says that the cardinal flower is one of those plants that few can winter even if it grows wild next door. Hope this helps

Peace Lilies, if one is referring to Spathiphyllums are tropical house plants.

Pothos, Epipremum ditto

Ruellia, also known as Mexican Petunias are also tropical but make great garden annuals in cool climates and will grow in a pond in the summer.

Caladiums, Callas, Cannas, Colocasias and Coleus are also not going to be winter hardy in zone 5.