Theory guy here.
- The amount that a fashion designer makes is dependent on their level of industry, cost of living, and location. According to the U.S. Bureau of Labor and Statistics, the average wage of fashion designers at all levels is $72,620.
- Since Rarity lives in a small town, the cost of living is very low when compared to a city like Manehatten (and its namesake), so this is probably why she is able to indulge herself in many of the luxuries she enjoys. Basically, Rarity can appear richer than she actually is. However, this factor also reduces her statistical income since every other pony in Ponyville's cost of living is lower, too.
- Thankfully, with the episode Suited for Success, we get to see Rarity getting attention from Hoity Toity, who seems to be very popular. It is safe to assume that Rarity is a cut and sew apparel manufacturer who statistically make a higher than the average wage, at $73,030. Because she is exporting her dresses to Hoity, her location doesn't mean much. This is a conservative amount, since it doesn't take into consideration money Rarity makes from ponies buying her clothing in Ponyville. It's a moot point because Hoity may not be buying from her year round, and she may very well be more successful than most fashion designers in rural areas.
- Following version two of this bits to USD conversion theory (one bit is about $3.50), $73,030 (the most probable average) equates to 20,866 bits when rounded up.
Tl;dr Rarity makes 20,866 bits annually.