Aloha Joshua,
Assuming this is a Vertex RO unit, not the RO/DI unit. right?
You did not say which filters you were planning to replace. If you are talking about the sediment and carbon filters, Yes, those can be purchased for under $10 at the common hardware stores. The 5 micron sediment filters that come standard on Vertex units are very common and 1 micron filters work good too. The Carbon filter can be either block or non-block. The block may last longer, but I typically use less expensive non block carbon filters for 6 months. which is about how long the sediment filter lasts. Here in Murray the water is so clean that my six month old sediment filter is still pretty white. That means it does not need to be replaced yet.
If TDS is above 20ppm, then it's time to get a new membrane.
Looking at their website, I see that all three Vertex units use 100 gpd membranes. A 75 gpd membrane can be used without any trouble and without having to change the flow restriction piece.
If this is an RO/DI unit, I suggest saving yourself some trouble and expense by removing the spent DI media and not replacing it. Our reef aquariums can easily handle that additional 1-5 ppm of impurities. Really.

After all, we sir into the purified water over 1000 ppm of salts, Calcium and trace elements, many of which were actually removed from the fresh water by the RO unit.

Mahalo,
Mark