Your analysis sis correct. The only way to come out flat or slightly ahead *maybe* on average is:
- know where you're going in the first place, wandering is expensive
- if you're doing a mod, buying a great working version of your base car will be a waste, buy a non-op with blown engine/trans, you'll pay less than you could sell those parts in most cases I think
- use a donor car in many cases of doing LS swaps (the bits you don't think about add up fast)
- do as much the labor yourself, with friends, etc., but use a pro for things out of reach, like quality paint, serious engine building (unless you're enkeivette)
- don't cheap out or half ass something, it will be used against you when comes time to sell
- be complete
- pick something interesting but potentially demanded. if it's too obscure you could be waiting for the right buyer for a long time. if it's too common, you'll be on the losing side of Supply vs. Demand. One potential hole here I see is older Mustangs (60s and early 70s) with the new Coyote 5.0 or *maybe* the Ecotec Turbo . . .
I probably have low 30s into my Vette. If I did it over, or (now!) I could probably do the same for around $20k, maybe less. Since it's a little unique, there isn't a market to judge, but I'd be surprised by anything over $20k. If anyone wants to offer $30k I'll sell it right now.
Generally speaking, you're cracked out if you think you're going to put together modded cars and make any money on the exchange. The guys who made money doing this, and there were a few, did so because they had a name and were selling a "brand premium". A no name could do the exact same thing and not get anywhere near the same price.