Bukkit

What is the issue?
Is there a way I can create a shop so that when they right-click a certain sign, it detects whether their money is above or below a certain price. If their money is above or equal to the certain price, it takes that amount out of their balance and gives them an item. Is this possible?

Please provide any additional information below.
I also have the plugin ServerSigns if this helps at all. The only thing I can't figure out is how to detect whether their money is above, equal to, or below the set price.

You can do that since a few versions.

Look at the documentation, you can detect right click in a general way.

Use it to check if player clicks a sign and if the sign has shop text on it.