As long as there is space for your investor to make their profit, your assignment fee is fine. Everyone wants to make as much as possible. However, no reasonable/serious investor is going to begrudge you your profit. And, yes, you can do a double close - if it's legal in your state - and bake it in. However, it will be disclosed in the closing details, so don't think you're hiding anything. Just be upfront with you investors, don't squeeze them and find good deals. Personally, if you brought me a deal that met my criteria, after your assignment fee was baked in, I wouldn't care if your assignment fee was $50k. The fact is, you left enough meat on the bone for me to get my profit.
I don't think that there is a limit on an assignment fee. However, on the other hand you should use some common sense. If you bring an investor a deal where she makes $20,000 and you're making $15,000 they may not like that.