I believe bits may be about 0.50 cent to 1.00 dollar. The industrial pet hair dryer the C.M.C. bought with the tiny gem that Rarity gave Spike I feel might be like 50,000 dollar. I not sure what gem it was but if it was a ruby it would be rare.

So what would be a good exchange rate for bits and gems.
1 bits = 1.00 dollar
1 gems = 1,000 bits or dollar.

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Gemstones are very common in Equestria, and are mostly valued for aesthetic reasons. With the number of gemstones under Ponyville alone, the fact that mines, or rock farms, are situated above massive caverns of crystals, and the Crystal Empire's own exports, I think a single gem would range from a few dollars to no more than a few hundred.

In Season 2 Episode 19, Putting your hoof down, we get to see the value of 1 bit. Background pony Poindexter purchases a bundle of asparagus for 1 bit without haggling, suggesting that this is an established market price. A bundle like that is approximately 1-pound, which costs as low as 2 dollars in USD. Some organic asparagus goes as high as 3.50, but since Equestria's agricultural industry is powered by magic, (and over 50% more land due to the limited livestock) we can reason that asparagus' value is on the low end. Additionally, though the wholesale cost of 1 pound of asparagus is around .64 cents a pound, (making your estimate more accurate), the character purchases only a single asparagus from a retailer, meaning he paid the consumer price, not the wholesale price. This makes the exchange rate: 1 bit = 2 dollars = 1 pound of asparagus. This means that later, in the same episode, when Fluttershy is asked to pay 10 bits for a cherry, the merchant is asking for 20 USD for a single cherry.

6525595
I'd also like to add that in that same episode, Fluttershy purchases 3 tomatoes for 2 bits. Though Pinkie haggles down to one, it's clear that the market price for those specific tomatoes was 2, even if that stand sold cheaper tomatoes before (It's rare that inflation doubles the price of things in a few days, so it could be due to different tomatoes, unusually low harvest, etc.). Prices for tomatoes range from 1 to 7 dollars per pound, and fortunately, 3 medium-sized tomatoes weigh about 1 pound. This means the price in USD for 1 pound of tomatoes was 4 dollars, well within the range of tomato pricing. Even Pinkie's amazing haggling skills still left the cost well within the range, at 2 dollars to 1 pound. Though I don't think the merchant would be happy about that.