In California, if you don't have a contractors license, your fee/payment should be less than $500. If it is going to cost more than $500, it's usually broken down into different projects and contracted separately.
Check what Oregon does. You can usually find out from a non-licensed handyman. He'll tell ya.
What I've found is that a set-up of sprinklers and lawn (no perennials, shrubs or trees) is usually around $2500, more or less depending on the contractor and what kind of deal you can make. Under-the-table work is usually $1,000 less but those guys tend to leave you with a mess or just take off with your money.
Please remember that I'm talking California prices. Oregon may be different. Your friend or you can call around to a couple contractors and find out how much it would cost. They will usually give you a ball-park figure if you tell them you aren't sure how much money to save up for the job. (And you may decide to do that!)
If it rains enough where you are that you don't need sprinklers, you'll probably still need to add some amendments and prepare the soil and you'll need to buy the grass (seed or sod?). Have your friend buy all the products you need so your bill will be straight labor.
To be certain you should do this with your friend, there are three things you can and should do. First, think how much is it worth to you to do all this work. Next, ask your friend how much he is willing to pay; how much is he thinking of. If those two prices are in the same range, the last question is the big one: If things go wrong, what will it do to your friendship?
Hope that helps!
PS. My nickname is 'Mama' Sharon because I get carried away.
