This should depend deal to deal. Pretend that it's you who's actually going to buy this deal when you're wholesaling. Would you take only a 10-15% return on your investment on a property that hardly needs any work done to it? Yeah! Would you take the same return on a property that needs a new roof, floors, drywall, foundation issues, etc. Probably not.
You also have to play your ARV really safe. There's only one place where you shouldn't be optimistic, and that's when evaluating repairs and resell values. You should actually be negative, take the average of your comps and go with a lower number, assume the repairs are more than you think, and this will enable you to apply these formulas properly and make your end buyers money.
Here's my company's formula for deals:
Properties that need no/light cosmetic work: 75% arv - repairs - your fee. This enables a higher price to the seller, enables an easy return for the investor. They can come in, clean it up, or just repaint a little bit, and sell it for a ~25% roi before closing costs. About 15-20% after. That is if they're going to sell it, of course. They could buy it at that price, refi 80% out of it and basically pick up a free cash-flowing rental. Sure, it's entirely possible to get the deal for less to make more money, but you'll lose deals doing this just for trying to take a little more equity and putting it in your pocket.
Example of the one above: $100,000 ARV home that needs $2000 in cleaning up. I would say offer $70,000, try to make $3000 on the deal, $2000 in repairs, sell it for $75,000 to the buyer. They'll put $2000 in, pay $73,000 for the house, sell it for $100,000. They'll make $27,000, and then pay ~$8,000 in closing costs. 19% ROI for hardly any work, but a little time, assuming they're cash.
For a medium-high work job, I'd go for 70% arv - repairs - your fee.
$100k arv house that needs $20k in work, I would offer $45,000. I would wholesale it for $48,000-$50,000, the investor comes in with $70,000 and will sell it for $92,000 after closing costs, or about a 31% roi.
I will say though, if you pick up a property on a REALLY good deal, lets say $100k arv, $10k in work, but you have it for $35,000, I wouldn't try to squeeze all of the equity for yourself. I would say you should go up to a 50% roi for an investor. To find this, so you can reasonably assign your huge deals (of course, I would recommend double closing if you do want to squeeze the equity for yourself. Then they won't see that you're making more money than them on the deal, but I think the investor deserves to earn more, personally.)
x = Purchase Price
y = Your Assignment Fee
z = Investor profits = ARV - x - repairs -.08(ARV) - y
z = 1.5[x - repairs - .08(ARV) - y] for a 50% ROI
This formula will enable you to give large returns to your investors for home-runs while reasonably capping it at 50%, for a larger assignment fee. For example,
$100,000 ARV $12,000 repairs $35,000 purchase price $8,000 closing costs:
At that, with a $0 assignment fee they would make a 82% roi. Well, now that you have the roi for a $0 assignment fee, you can subtract 50% from 82% to get 32%, which you can then multiply by .5 (because you want it to be a 50% roi for them) for their initial investment for your fee. $55,000 x .32 x .5 = ~$9,000
You can check this by taking what they spent vs what they will sell the house for. This means you'll make $9,000, they'll make a 50% roi on their money. I only offered this solution because some investors will care how much money you're making, for whatever reason.