Crystal Quest makes a "Portable Countertop" reverse osmosis filter that is rated for 50 gallons a day. It comes with multiple fittings so it fits any sink. My friend got one for $125 shipped 6 months ago, and since then we have used it for both of our grows AND about 50 orchids. It brings tap water down to 2-4 PPM from about 150-250 PPM tap so really close to 0 PPM... I actually went and bought the exact same one..found it for $115 shipped from a blujay.com seller but I would recommend going with a more well known seller as my guy has taken 3 weeks to finally ship it...I'm expecting it tomorrow via fedex. Same filter as my buddies.. haven't seen another RO system under $200 besides this one...and this thing is crazy, he can take it off his sink, put it in his backpack, give it to me inbetween classes, I bring it home, plug into my sink and filter out 15 gallons for my plants, and bring the filter back to him the next day in my backpack.
it basically screws onto your existing faucet and the filter itself sits in the sink or countertop, with the waste water going into the sink drain. takes 10 seconds to take on/off and no plumbing. exactly what you are asking for.
I bought a 4 stage system like this last year for around $80-90. I'm really happy with it. //cgi.ebay.com/PORTABLE-REVERSE-OSMOSIS-WATER-FILTER-SYSTEM-DI-RO-105G-/140482170989?pt=Small_Kitchen_Appliances_US&hash=item20b563d06d
Just got a portable one from purewaterclub.com that screws into the faucet. 4 stage filter and a TDS meter for $ 83.00 ( no tank ).