Welcome to HuntChat, Rover. The answer to your question depends on what type of clays you want to bust. I you and some buddies want to get a thrower and do it yourself, it doesn't much matter what you use. If you want to to go to a range and shoot skeet, trap or sporting clays you may want a different shotgun because doubles are hard to do with a single shot.
If you looking for a shotgun a pump or semi is going to be the cheapest. Semi's are more popular on the range but a pump will do fine. Check the used gun racks. A decent gun can be had for $200. If you come across a J.C. Higgins 12 ga. pump in good shape, buy it. They can usually be had in this area for about $100 and are worth every penny and then some.
For ammo, unless you reload, get the cheapie "gun club" shells. Not counting the cost of reloading equipment, you can reload a box of shells for about $2- $2.25. Buying cheap shells are about $3.50 a box. You have to decide if buying the equipment which is about $100 is worth it to you.