Bagged compost is reasonably priced, too. For in-ground plantings I just mix compost in with the native soil (mine is clay, too) at planting time. In a raised bed situation you can do the mixing together ahead of planting time; just don't forget to mulch. That newly perfect space will be a magnet for weed seeds otherwise, and they always grow faster than our desired plantings.
For the most part, a raised bed is a space whose planting medium is fluffy and loosened -hence raised. When you dump in topsoil, compost or other materials you're in effect just emulating what could also be done simply by digging up and loosening your native soil. Adding compost will help it drain away excess water, while also helping to retain water during dry times. The sides of the bed are simply there to keep your loose growing medium and mulch from washing away in a heavy downpour.
So don't over-think it. Keep it as simple as possible for your situation. If you can't dig your soil at all go with more new medium by volume than you would if you could dig. If you
can dig your native soil (a day or two after a good rainfall is a good time to check for this
), you could just mix in compost and plant.
