| seems to me that some kind of envelope filter like a q-tron is the right direction - the q-tron is really flexible and sounds great - but at least half the settings won't be ideal for bass, and the volume spike is serious enough that you'll find yourself in need of a limiter as well. Keep that in mind - many envelope filters have such a loud volume spike side effect that they can put your ears and amp at risk without some kind of limiter - and that's an additional expense.
Here's 2 ideas for you: One way to go would be with a DOD FX25 or 25B. These pedals range from 20-50 in price. They are no longer made but are very common. They have a distinctive character of their own, and can do nice smooth funky sounds as well as deep, booming dub tones. The FX25 is arguably the envelope filter of choice for modern reggae (not necessarily for roots). Its amazingly cool for the price and pops up in the tb classifieds all the time. However, it requires a limiter because like the q-tron, it has a pronounced volume spike that needs taming at some settings. The fx25 is blendable (and generally cheaper) but many people still prefer the original fx25s (partly because the old ones were USA made). For a limiter i would get a Boss LMB-3 or a guyatone BL2 if you can find one. Neither of these should cost more than 50-60 bucks. So with some patience you should be able to get an FX25 AND a limiter for under 100
A more expensive alternative would be an EBS Bassiq. It isn't very celebrated in the fx forum because it only has a threshhold (sensitivity) and attack knob - 2 knobs! But with those and a 3-way switch (and an internal blend trimpot too), I think you it can get a HUGE range of sounds, from subtle "bottom rocker" effect (for rock) to mu-tron/q-tron swampy thick (for funk) to pumping subharmonic swells (for dub) and anything inbetween. not too many simple, compact envelope filters include an attack knob. You can actually use it to do "slow gear" style volume swells and moog-ish "tom Sawyer" effects, too! (though some would argue you need to chain 4000 bucks worth of synth pedals to do that last part). It can be found in the TB classifieds, ebay, etc for around 120-140 a little over your budget, I know, but I got mine for 110, sometimes you have to look around. It has a more controlled volume than many other envelope filters - I can't say you'll NEVER have a spike or you will never want a limiter but you wont need one right away. EBS pedals aren't always the most far-out and tweakable, but they are designed for bass and have useable settings up and down the dials - they're practical , pro-oriented effects rather than tweakboxes where you'll never be able to reproduce the settings. (note: I AM NOT A PRO haha) | 
