Petites can vary widely in price depending on who's looking for them at the time. I think of any pony set, petites have the craziest price fluctuations. I've gotten into bidding wars before for that last petite I needed for a set, and I've also gotten them dirt cheap in lots. The most I've paid for a single Glowing petite was over $70(much to my shame - it was a bidding war, so I wasn't the only one willing to shell out. It was a really nice petite and the last one I needed from any set!). I've regularly bought individual petites to fill in gaps at the $15-$25 range, but for the higher range they better be in excellent condition.
Glowing and Twinkle petites are the most likely to have a lot of interest. The Bright Sight ones seem to have fallen in popularity a bit.
Last time I checked, Bright Sight petites on ebay have sat there, unsold, at the $25-$35 each range. I do see some of them snapped up in the $10-$15 range, so I think that's about what they're worth at the moment. I haven't been in the market for them for a while, so I'd check completed auctions to see if any started selling higher recently.
The Twinkle and Glowing petites - I'd say $15-$35 depending on condition. The better the symbols and hair, the more you can get. A truly mint one can go over the high value, sometimes by a lot. It's rare though. There are a few petites that have such delicate symbols that they rub if you look at them funny. A mint one of those can go pretty high.
Fyi - petites make *excellent* trade fodder, and if you're interested in trading, you can often get more bang for your buck as they say:p A lot of traders are more forgiving of condition issues in trades for some reason too(just make sure you are clear about the condition - more often than not it won't be a deciding factor). They're also easy to store in large quantities, so I generally like to keep my petites as trade only. If you're really looking to sell, you could always take offers on your best ones.