Ultimately it's worth what somebody is willing to pay. I'm far from an expert, I don't actually buy a lot of burl but I do price shop online quite a bit. In my experience common domestic burls that size would run $20 - $40 (like maple or cherry), common exotic burls or less common domestic burls would run $40 - $80, and rare exotic burls that size would be $100+. Both species and quality are important, a really rare burl like cocobolo won't need a lot of figure or eyes to get a premium, but a piece of maple better be super figured to fetch top dollar. Average HRB in that size would probably run around $100, and premium stuff would be a bit higher.
All that said, based on the photo I would be skeptical that it is HRB. Wood sellers and resellers have a lot of incentive to incorrectly identify wood, a problem that only gets worse with burl. For example there is a reseller on eBay that often sells blanks he labels as "desert ironwood burl", a quick search shows that it is not actually DIW, his response to inquiries is basically "the guy I bought it from calls it that, so that's what I call it". It is possible that the HR label was applied by an unscrupulous reseller... In my opinion the block that you have looks more like coolibah than HRB.
Again, this is all just my opinion and I'm not an expert by any stretch. So take it all with a grain of salt.