uni-panther is wrong. Most subwoofers I've seen have LFE inputs now, even in the $200 range. If it shows it has a LFE input, it has an LFE input. Many subwoofers have them (at least any of them I would consider buying in the price range.
Some subs without a dedicated LFE input will let you use the standard input by just feeding 1x rca to it instead of 2x (ie just one channel) and then turning the crossover on the subwoofer all the way up to max to bypass the internal circuit. You need to check around for the specific subwoofer if it doesn't have LFE whether it has this functionality. It is not simply a matter of turning the crossover high enough that you're pushing control of the full range to the receiver, which sounds terrible. Some subwoofers without LFE inputs are designed such that when you have 1xrca and the crossover maxed it will actually bypass its own internal circuit and act as an LFE (for this reason it has to be maxed on the crossover as anything less than turning it all the way to the right will keep it on the internal crossover rather than LFE). This is mostly for subwoofers within the past couple years and you need to check either online or with technical support from whichever company you're looking at. It's mostly cheaper subwoofers that are this way (or possibly older ones) so generally I'd say to stay with a sub that has a dedicated LFE, but if you're just looking for a subwoofer with reasonable sound and not very good sound, those kind of subs will sound fine "LFE"-ed.
I'll post back when I have a chance with a bunch of good choices for subs in the $180-350 range that have LFE inputs. The rest aren't necessarily bad, but I don't really have the time to check on which models can be "LFE"-ed or not. I know some can, some can't. If they can't, they aren't worth your time. If they can, they represent be a good budget value if you aren't looking for something fancy.