Local garden centers - some order (bulk) and repackage seed cultivars which have proven to do well in our area. They also carry multiple name-brand seed collections.

Lowes and Home Depot - general stock a collection(s) of name-brand seeds, one of which is Burpee. Both return the seed racks to the vendors at the end of the spring growing season. So plan ahead for fall seed planting needs.

Walmart - good collections of open pollenated seeds, inexpensive, few hybrids... Might satisfy some of your choices, depending on your cultivar preference. They also send back their seed racks.

Dollar stores - lowest prices but seed count is less. So similar in value to Walmart but with far less cultivar selections

Mail order - lots of choices with many providing some targeted interest, e.g. heirlooms, tomatoes, melons, etc. You will definitely find best quality with this option but typically (meaning not always) at a higher price and with the usual shipping and handling charge. .. S&H is easily justified if placing a large order. Most mail order seed vendors will periodically offer discounts.

So, my answer to the "cheapest and best" question would be all of the above...

I just returned from one of our our local garden centers with their repackaged Premium Crop broccoli seed. Mail order was the other option but this time I paid in gas vs. S&H to get the seed.