How handy are you? For the price of one of those kits, you could choose some inexpensive tile and make a shower that would last a lifetime and might even increase the value of the house rather than decrease it.
Seriously, if you don't go overboard with fancy tile, you can find stuff that is less than $1/sq foot. Some cement board, plastic, a liner and some thinset and deckmud, and a drain, and you could get a solid shower for probably in the $300-$400 range. Cement and mortar are pretty cheap. You could use a shower curtain and spring for a glass door later, if you wanted to. Most shower kits would cost at least that. You can get a shower valve cheap, if you can't reuse what is there (recommended, and maybe required by local codes to come up to current standards). You can get a neat hand-held wetsaw (sort of like a circular saw with a hose attachment) for about $60 including delivery on special from Felker if you check the site below.
Check out www.johnbridge.com for help, if you want to look into this possibilty.