Steamers are good for ovens, but first use a chemical to break up the grease. It'll make the job a lot quicker.
Cillit bangs good stuff, or F.E.C. if you're going to be doing a few of them.
Wear a mask too, cos sticking your head in an oven full of chemichals is'nt reccommended till you get your tax bill.
For pricing, take into account the time to take the oven apart, clean the elements and underneath, getting into the corners, and cleaning the kitchen sides up after.