I bought a system from Machine DRO to replace an elderly Anilam DRO on my Smart and Brown. One of the scales had failed but I decided to pay a bit more and replace the lot. That was a general-purpose kit which came with a whole heap of different mounting brackets but I see that they sell a few dedicated kits for the Bantam (see here). They have something that's about your budget.

I also use 5 micron scales; that's certainly good enough for my fairly elderly and well-worn machine but when I translate that back into units I understand, that's still only two-tenths of a thou. OK, that's four-tenths on diameter but that's at least as good if not better than the handwheel scales. Chasing to the nearest micron is a bit more than my machine justifies and I guess the same is true of the Bantam (great machines though they are - I did a lot of my early work on one of these).

Based on my dealings with Machine DRO, they are probably good for making something up if you wanted a different make of DRO or whatever - they seem pretty reasonable to deal with. They are also UK-based with a real address and everything - can't say that about a lot of eBay deals! But you pays your money and you takes your choice.