Cornucopia: Can Horn of Plenty be used together with normal treasure cards?
Horn of plenty doesn't give you money, it lets you gain a card costing up to the number of different named cards you have in play.

 

This means you should always play it after your other treasures, you then gain a card from Horn of Plenty. Afterwards you then do your buys as normal. Horn of Plenty doesn't use up a buy or any coins. If you don't want to buy anything afterwards, you don't have to. Playing treasures gains you coin to spend, it doesn't obligate you to buy something with them!