The fact that this is even brought up shows just how inexperienced this poster is.
It goes like this, if you can find a sucker to pay unique prices for a rare good luck.
It's more the case of the unique only being worth as much as the rare, not the rare being more valuable than the unique.
The situation the OP is talking about is an exceptionally good rare with range versus an exceptionally bad unique with range. I've seen them, where they have a massive but ultimately useless boost to hit chance -- and that's it.
At level 50 with the stats provided:
The rare is doing 159 DPS, range 37.7
The unique is also doing 159 DPS, range 36.97
If I had both guns in my inventory I'd equip the rare over the unique, but that doesn't mean I'd pay unique prices for the rare. If someone was trying to sell me the rare I'd give them a deal based on it being a rare. If someone was trying to sell me the unique on the otherhand, i wouldn't give them unique prices either cause well, it's a pretty crappy unique. Heck, if it wasn't upgraded, I'd probably give them a worse deal than the rare since it would cost me more to upgrade than the rare.