We use a split approach with 0.5% paid to outside sales (ie. outside real estate agent) and 1.5% paid on inside sales (ie. no outside real estate agent). We also offer sales incentives akin to bonuses for targeted performance (ie. specific communities or specific targeted sales goals) as well as a team commission of 0.9% paid to all sales representatives to promote greater teamwork. The team commission essentially creates a base for the new agents. We also pay for E&O insurance, in-office training and professional fees.
Many savvy agents may only be attracted to your offer if it allows them to concurrently maintain a hand in thier own marketplace. If that were possible, you might find a savvy agent willing to work your project if your area is slow.
Builders around here normally pay about 1% of the sales price. So if the units are about $300,000 that would be about $3000 each and they'd need to sell 23 or so to hit the $70,000 mark. Can they sell 2 a month? I think it just depends on the marketing plan for the building, how many units you have and how long you expect them to stay. You have to figure out a way to keep them there through construction and closing, but perhaps spread their payments out over that time period.
If you do upgrades, 20% commission on the upgrades.