I bought my daughter an ATI Kofs 20ga. O/U last year. They have a few different models of 20 & 12 gauge. I found the one I got her in a local shop for under $500. It has the extractors, not ejectors, but that's okay.
A friend of mine that has Perazzi's once told me that the big difference between O/U shotguns is the durability. For a shotgun like the one I got my daughter, taking it out to shoot a few times a year or even once a month is just fine. The more expensive ones are those that can hold up to 100's or even 1,000's of rounds a week or month.