Up until last week I had been using a Tamron 90mm and although some stealth is needed the lense was pretty sharp -
I've now picked up the Sigma 180mm (secondhand) and am finding the extra working distance a breath of fresh air. With butterfiles you can get some excellent pictures without needing to be right on top of them but you can get in closer if you wish. Secondhand though this was slightly over budget. Judging from my experience using both a smaller and larger working distance I'd say a Sigma 150mm would be great if you can find one s/h.
Good luck!
TobyH