Unlike tube guitar amps, you don't have to crank a bassamp for it to sound good, but you may just want to when you want to feel the bass.(just turn it down when you don't want disturb the neighbours) so there isn't a real need to aim for low wattage. The only practical considerations are budget and size/weight.
Personally, it would be good to aim for 35watts or more so that you will have a little headroom for the times where u want to practice to a drum backing track input or perhaps jam with some friends in a home studio setting. You might outgrow low powered bass amps pretty quickly.
I think the Beta-Aivin BP35 at standardvalue sounds pretty good, but its above your budget.