is it a single or double speed one? you can see the difference on the large hand wheel, and whether it has one or two sets of teeth on it.
The 2 speed ones like this one
http://www.ebay.co.uk/itm/Bench-Drill-U ... 500wt_1277 tend to fetch somewhere around £25-30 on ebay. sometimes you can get lucky, because they are typically heavy and collection only. You can get them for less if luck has it that nobody else wants one near you at the time. If I recall my research, I think that these are the Union 2x model.
The one speed like this
http://www.ebay.co.uk/itm/union-hand-dr ... 500wt_1277 tend to fetch a bit less, maybe £15-20. Note the simpler large wheel and the difference between that and the link above. Again, you may get lucky. I have one of these, and I thing that it is brilliant, although I would prefer a 2 speed one!
Union do seem to fetch a small premium over other brands, many of which are not branded. The other ones that seem to fetch a premium are those which look special, or unusual. You shouldnt need to pay a fortune for a Union one- there are quite a few about if you are patient.
HTH
Mark