I disagree with the "if you have to ask" philosophy. If you don't know something you have to ask (or read) about how to do it. You can't know it without asking, so hey, if you want to learn how to wrench your own bike, and can see changing bottom brackets & cranks again in the future then do it yourself. If you prefer to let someone else mess with it, pay the shop.
As others have said, you need to know what is included in the $100. The work involved is (assuming you don't change the rear derailleur):
Remove pedals, Remove current cranks, remove bottom bracket, install new bottom bracket, install new cranks, install pedals, adjust derailleur limit screws, adjust derailleur cable tension.
If you are also changing the rear derailleur: Break the chain, remove derailleur, Install new derailleur, thread chain, reconnect the chain, adjust limit screws, adjust tension, adjust B-angle screw. (assuming your derailleur hanger is not damaged)
This should take an experienced wrench about 30 min (45-60 with rear), so a "fair" labor charge is somewhere around $30 - $40 ($60-$80). These may actually be a bit high. A full build at my local LBS only costs $100, but I'm not sure if that is only if you buy the frame/components through the shop.
If you do it yourself, you will need the proper tools. If you don't have them already, it will be comparible in cost for you to do it yourself vs having the shop do it. For example, you will need the following tools:
Bottom bracket tool (of the type to match the bottom bracket currently on the bike & the one you are installing. They may be different). ~$15 ($30 if you need 2 types)
Ratchet or crescent wrench to utilize the BB tool (Assume you have one)
Pedal wrench: ~$10-$15
8mm crank-bolt hex wrench. (you need a longer handle to get the torque right). ~$5
Grease (for the threads of the pedal & BB (depending on the type, you need anti-seize compound for a ti BB). Item on hand or you need to buy a tub. ~$5-$10???
Screwdriver (to set limit screws). Item on hand
5mm hex wrench (to adjust the derailleur cable at the front derailleur) ~$1-$2
Good maintenence manual (Zinn or similar) ~$15
Maybe (probably) a torque wrench: ~$50 ??? Just a guess.
So the decision should not be based on cost, since they will be similar. It should be "do you want to do it yourself to learn more about my bike?" or "do I just want it done and I'll probably never do this again." If the former, by all means do it yourself. If the latter, have the shop do it.