I love that nemo bucks can be broken down into cookies in case there is an item you want that is the other type of currency. However I understand the discontent with some of the high prices of the premium currency items. I think you should be able to make the cookies exchangeable for bucks as well. Of course it wouldn't be an even exchange, but I think it would help alleviate some of the pressure on the microtransactions as they are very heavy in a non ftp game.
Other stores a lot of times will offer dual pricing for items which have one price for the 'game' currency and another for the premium. This is also a good solution however, since the exchange feature is already a thing from nemo to cookies, it makes sense to allow it the other way.