This is as simple as this - find out your hourly price and stick to it!
When you get such an offer, analyze the project in detail and calculate how many work hours it will take you to finish it. Then tell your price to your "people". They can add their commission on top of your price.
If you want to go deeper, you can calculate your maximum hourly rate (for short projects) and your minimum hourly rate (for projects over X work hours).
But basically, it all comes back to you.
If you are not sure about your hourly rate, then browse statistics on popular freelancing websites, as they will offer the average hourly rate for each field. After that, try to figure out if you can use the average rate or you have to (de/in)crease it.
Also, don't be intimidated if they tell you "Can you do this job for XY dollars"? Simply analyze it to see how much work there is, and then calculate the hourly rate. In a matter of a seconds, you will know if you should take the project or not.