Pure dps in that budget, it has to be BLP and unlimited, and sib.
The only thing that comes to mind is a Ranked Flamethrower. Use any cheap blp tagger and you are good to go.
T0 RF + Beast = 53.55 dps @ 2.911 eco
T5 RF + Beast = 73dps @ a ton of enhancers burnt
Side note: You cant hunt underwater mobs ( Levithan etc ) with this, just so OP is aware of this limitation.
Ozpyn BP S1X4, little less dam & eco than the flame thrower but would work well.
Side note: You cant hunt underwater mobs ( Levithan etc ) with this, just so OP is aware of this limitation.