I used a 10 G tote tub for my RO water storage approx. $10 at walmart or Canadian Tire and a DIY Aquahub kit from BRS and it has everything you need for building your ATO, 2 floats, power relay and assembly strip $39.99, a small gph pump I paid $15 if I remember correctly. some misc. tubing and wire I had on hand. So for under $70 bucks I have a great non- fail ato system that I've been using for about six months now. I'm sure you could hunt down a container of some sort for water storage that will fit in your space without selling the farm.
Here's a couple of pictures of my set up.
Not real fancy but it does a good job for me, so I guess it depends on how much you want to spend. I tried a couple of the other more expensive ato systems and nothing has worked as good as this one has for me.