Cheap but good quality water-cooling unit

As the title says, anyone got any ideas? 

Define "cheap"

I think the Kraken x40 presents a good value.

As far cheap, I don't think you'd get much better than the Water 2.0 Performer from Thermaltake.

But there are better water cooling units, and even air cooling units. I think the slim 120mm all-in-ones are really only good for tight spaces, but that's just my opinion, and probably a dated one at that since these smaller units have gone down in price. At $60, definitely go air over the slim 120mm units.

Don't get a CLC; the performance is underwhelming. I would rather get a Phanteks PH-TC14PE, if I were you. Better than any single radiator CLC out there.

Water 2.0 from Thermaltake is not bad at all, especially the Water 2.0 Extreme shows very good results.

Thermaltake has a long-standing experience with water cooling, they started selling DIY packages at times where everyone was using a garden pond type pump, and it actually worked very well. I personally would put more trust in a Water 2.0 than in a Corsair cooler.

However, get the right prefilled watercooler for the right reasons: if it's for overclocking, you'll need a thick 240mm radiator or you'll have a lot of work modifying a 120 or 140 mm radiator to make it perform well enough for worry free overclocking, you'll need aftermarket fans or extra airspeed (more noise) or a few hours of lapping the CPU block, etc... these smaller rads are a practical solution for compact builds or just for looks, but not a practical thermal solution.

Cooler Master Seidon 240.

Well since I have no idea of your budget and case I would say something like: Corsair H110, Corsair H100i, Thermaltake Water 2.0 Extreme 240mm, NZXT Kraken X60, NZXT Kraken X40, Corsair H90, Corsair H80i. It also depends on what you need it for.  If it is just instead of the stock cooler for some core i3, then just take something like the H60 if you really want watercooling, but if you just need that, get something like the Cooler Master Hyper 212 Evo instead, as it is much cheaper.

"Cheap" as in it won't blow my budget over $100.