In answer to bear on building cheap robots: It always depends on where the bits and pieces come from. Good example: I've got about fifteen steppers lying around on my bench. They came from scrapped floppy drives. Cost? $0 monetary outlay, though as you point out a fair bit of time was spent removing them. Another example: Last time I took trash to the transfer station, someone had dumped a bag full of R/C toys. I picked 'em up. One just needed a battery charge and some contact cleaner on the power switch. (Free toy for the kids!) Another had a fried transmitter. Question is, do I fix or replace the transmitter, or just gut the car and stick a microcontroller in it? $0 monetary outlay, and not much more than that for time invested so far. Offroad chassis for a RoboMagellan project? Possibly so. (Though my kids would kill me if they heard me say that about "their" toy.)
Problem with this approach is it doesn't scale. If you're doing commercial robots, you can't rely on surplus or salvage. This is one reason why commercial robots tend to be a lot more expensive than what can be built at home, even given similar construction methods. It also doesn't give any gurantee of identical spares. In that stack of steppers on my bench, only two are of identical make and model.
One more question to ask when estimating cost: Do you count the cost of items previously purchased? For example, the line follower I'm building is getting its entire drive train out of spares from my ROV and mini-sumo. Total cost? I could say zero since the parts are just lying around. Total cost for someone trying to duplicate what I'm building? Considerably more, especially since I didn't buy the motors as single units.
I guess what all this boils down to is this: Do people estimate cost based on their out-of-pocket expenses, or do people estimate cost based on what someone else would have to spend in order to reproduce what they built?
Even putting it so simply still leaves you with hairy questions. For example, do you add in tooling costs? I've got a complete machine shop at home and a decent electronics shop as well. I don't add in machining or tooling costs, but someone trying to reproduce what I do would have to.
I'd be curious to hear what other people's practices are when they estimate cost on a project.