You question is clear but I can't help wondering what you aim to do, i.e., what is the driver if there is one that promps you to treat the water. Reverse osmosis and distillation are different concepts from conventional filtering and the cost of a lot of the systems reflects multistages where the systems are doing different things. Right off the bat if I had to answer this, I'd say, forget the RO system, they are very wasteful and only worth it if you have no other choice (personal opinion, not to offend anyone else's tried and true methods which may be better than mine). I'd say consider a real absorption filter, get a good quality activate carbon one which will meet most of your needs, except removing Calcium and Magnesium if your water is too hard. I guess you want a specific link, but it is better I just mention Brita as on of the top three brands, and you can Google how to pop one open after it's used and refill it with activated carbon cheaply, or even buy a cheaper generic housing to start with and dump the cheap carbon in youself. It may or may not meet your need, but it certainly will meet your budget for thousands of gallons per change and can be helpful for many hydro systems.

I've always wanted to build a big sand filter to see how well it would work.

If you are only looking to spend less than $100, you won't really find a reverse osmosis system, and if you do, I wouldn't trust the quality. But there are systems like this one on Ebay, http://www.ebay.com/itm/REVERSE-OSMOSIS-WATER-FILTER-5-STAGES-AQUASAFE-100GPD-/260867033764?pt=Small_Kitchen_Appliances_US&hash=item3cbce30aa4 from Aquasafe Systems which is a little more than $100 but is a good quality product.

As for the comment talking about a Brita filter, why even waste your money, just leave your water sit out over night, and you get the same results. Activated carbon doesn't do anything to the water except remove chlorine, and for the Brita for example, removes about 55% of it.

Here is a link that might be useful: Aquasafe Systems

The above commentor does not understand that activated carbon effectively removes a host of organic contaminants which the EPA considers toxic, odiforous and off-taste. Ignore the 55% comment unless a proper reference is provided which can be interpreted for your particular use.

Also, perhaps in Canada chloramines are not used for disinfecting water, but many US utilities do use them. Chloramines, unlike hypochlorination, persist and cannot be removed simply by airing water out overnight.

Activated carbon filters for this reason are common in aquarium systems as well. That is not to say one will fit the application either. But it will fit the budget.

The Canadian seller the poster is promoting is nothing remarkable, just trying to meet a low price point, but still not not within the budget requested. It has already noted likely has both ongoing replacement costs and is very wasteful with water as are all RO systems (for every gallon made three are wasted).

What matters with regard to selecting whether a filter can meet the OP's price point and need depends on the application, which was not stated. If the RO system is needed. Don't forget that most of the RO systems take a bit of effort to install and you likely need to budget a bit more for supplies. They are great is you need one, though. I don't since I just collect rainwater off a tarp I set up and get free rainwater with 1 ppm TDS (0.002 mS/cm) , but this is for my hydro. That is better than the distilled water and purified water being sold in the stores and machines around here. My Tarp cost $7 at Lowes. That is another alternate that makes the budget if you can use it.

I have one of these hooked up in my basement to a 30 gallon tote reservoir (link below).

Been using it for 5 months. Works great. I'd recommend it.

Here is a link that might be useful: $60 RO system

That last system is the cheapest I have seen.

We looked into it but being a bottom line guy didn't feel that the benefits warranted the cost. Never had a growth problem all summer out of the hydro system.