Interesting. So the price ratios get higher the more useful the effect is.
In that case, a +1 card project would be at least $9, and I might even put it at $10, since +1 card is even better when it's at the start of your turn.
Considering Poacher is $4, a +$1 project would be $6 or $7. I'd put it at $6 since $ is much less useful than actions at the start of your turn.
Also, Cathedral could probably stand a price raise. It gets docked a few $ for the non-optionality, but I still think it could end up higher when compared to the others.
What other vanilla bonuses are there? Hmmm... what about a +1 VP project? The closest we have to cantrip +VP is Groundskeeper, but the real deal is probably better. I'd put it at $9, since +VP isn't useful at the start of your turn.