What model of Lawnboy is this? To do the bottom seal, you should unbolt the engine from the deck and flop it on it's side. CAREFULLY hook the seal and pull it from it's recess in the case. DO NOT scratch the sealing area in the case. Clean the seal seating area. Clean the crank shaft with scotch-brite or light sandpaper to keep from damaging the new seal when you slide it on. Put some oil on the newly cleaned shaft and slide the seal on. Take a piece of PVC pipe of the proper diameter to fit over the crank shaft and tap the seal into place in the crank case recess.

Oops, sorry. I didn't answer your price question. Seals should run about $3 to $5 each.

Here is a link that might be useful: Seals

One thing that can stop this show is inability to remove the blade driver which prevents seal removal or installation. On a self-propelled, the drive pulley also has to come off which may or may not be possible in the back yard. Assuming you get both of those off, you use emery cloth strips on the crank to clean the whole thing or else the new seal will be ruined before you get it to the block. After polishing the whole exposed crankshaft, clean it with solvent to remove any grit remaining from the emery cloth. THEN you can remove the old seal, pull it off the crank and install the new one.