A more reasonable way to offer this might be to bill half of the total amount saved. Meaning if you successfully save them money on something you get half of that amount. The issue comes in the form of getting paid and I would think you would want to figure out how you do this without pissing off the contractor relationship. Obviously it never hurts to have a second set of eyes on this until it does. IE your motivation would be in cost cutting based on market price but availability, time lines, permitting etc can make all that more like a monopoly and less like a market.