I have a Yanmar RS1303 that I use on my
B7610 (which is basically the same as your
B7510, just 2 HP more at PTO).
These are typically available for $300-$500 used, and are a good option if you are looking for something inexepnsive. Just be sure to make sure the tines don't need to be replaced, as a replacement set costs $200. I paid $200 for my RS1303, and put $200 worth if tines into it and a $15 tine shaft seal. It works great.
To use one of these Yanmar tillers on your
B7510, you'll need a set of longer (32-35") lift arms to get it to sit at the correct angle, and also to give enough driveline length to be able to put a slip clutch in. If you need to know a source for these, I can look it it for you. I paid about $50 for the pair. They're easy to swap out for the stock ones (which I put back on for anything else that I use on the 3PH). A hydraulic top link is also handy (which I have) for being able to get the right angle.