A good LA quilter will work with you but you must have reasonable expectations. Most custom LA work starts at around .025 to .03 per square inch and that is a starting price. As Dunster said it varies from area to area in the country. It also varies for hand guided vs. computer guided quilting.
You say an arm and a leg and that is relative. If you are working with say a queen size quilt measuring say 85" wide by 100" long at .025 per s/i you are looking at a minimum of $200 plus for the quilting. Most e2e quilters start at around .01 to .015 per s/i.
Things that take more time and attention drive the price up, like any amount of ruler work for stitch in the ditch, thread color changes, dense background fills etc. Award winning quilters charge a premium because they can (like Karen McTavish, Sharon Schambers, etc). If you find a bargain basement LAQ that will do custom it could be they are a beginner. Nothing wrong with that as long as they fully disclose that fact and you can see samples of their work.
What you need to decide on is a budget. Then search the board and internet for pictures of longarm quilting. When you see something you like reach out to the quilter and ask what can they do for your price range.