Generally speaking the cheapest aircraft to learn on are cessna 152's
You will note the use of the word generally in my post.
One of the most common training aircraft most people encounter is a C152, and compared to it's contemporaries it is also quite cheap.
However, it is not the cheapest, sexiest or most capable trainer out there, not by a long chalk. Hence I said generally.
As per my first post, my advice still stands to visit the clubs in an area first and foremost. Only then can you establish what they fly, how much it really costs and more importantly whether you want to learn there.