$25 for fork installation? Sure, why not. This is the problem with doing most of my own work, I never know what the heck to tell people a shop is going to charge them. I work for beer most of the time, and I find that the shops don't seem to charge twice as much for what I would consider a 12 pack job (fork install like this), as they would for a 6 pack job (my shifting sucks, can you fix it?).
Something you should be warned about is that shops can be a little bit cagey about installing parts bought elsewhere. The expectation of selling you the parts is built in to a lot of their installation/repair rates, so they might want a little extra if you BYOP. This is kind of how it goes, you should pay them so long as they are straight with you up front, but if they want to lecture you on the moral virtues of local bike shops and how you are killing them by shopping online, tell them that attitudes like that are why people buy bikes at the 'marts and sporting goods chains, and take your business elsewhere. I'd call them for a quote before you started messing around looking for a cheap fork on the net anyway, as you might not end up saving much by the time you find one, get it shipped, etc...