1) Twilight - The Princess of Friendship is most likely the richest of the group. Even though her castle was created by magic and not bought, it is still a sign of her status and must be worth millions of bits. Unknown if she has access to government funds or has a salary.
2) Rarity - Owns multiple stores that cater to both the casual fashion pony and the rich and famous.
3) Rainbow Dash - Owns a very luxurious house. Is weather manager of Ponyville and a Wonderbolt, which is an elite group of flyers. Both jobs probably pay well.
4) Applejack - Her family owns Sweet Apple Acres. In terms of property value, I think AJ is richer than Rainbow Dash. But operating a farm is not cheap, and a bad season could spell trouble. I believe AJ has very little left over when expenses are considered.
5) Fluttershy - The only reason I put her above Pinkie Pie is that she has her own house.
6) Pinkie Pie - Lives in Sugarcube Corner, her work.
What are your thoughts?