Cheapest PCP air gun?

I reckon that the cheapest PCP would probably be a Chinese made "78" CO2 rifles that has a filler cap fitted to the cylinder end so that it can be used with compressed air. The only problem, is that the rifles need work done to them .....so first you need to know what you're doing. A polite summary, is that they are basic and have limitations. I use one as a fun rifle in the woods behind my house. Can be pushed to 35ftIb in .22 (max of 5 shots) but have mine set far lower for more shots. It works and its cheap.
 
if you are looking for budget priced pcp, figure in to the equation price of the pump, and realize you are probably not going to like the pump. So you might want to figure in a bottle set up. From what I have found minimal bottle set up is around $300.00. A pump is around $150.00.

If this is already getting out of your price range, you might just want to stick with getting another springer.

Wal-mart has the Benjamin disco for around $225.00, and the marauder for around from $452.00-$530.00 depending on caliber and trim option. I also want to point out that if you are looking at entry level pcp's one thing might be the pistol/carbine option. Depending on your applications, this might work. You aren't going to get the air capacity, or range out of it, but you can still get 50ish yards out of it, plus it is shrouded. So it is very back yard friendly! Also look in the used or referbished market!

http://mobile.walmart.com/search/Bejamin%20maruader

so that would be the Crosman/Benjamin promo.
another really good name in the same-ish price level is Hatsan. Like Dan25 pointed out. You can get a Hatsan At44 for less money than a marauder, and it has the same if not better accuracy and range. I myself purchased an At44 long quiet energy model in .22 cal. off of amazon for $420.00 plus $30.00 shipping, for a total of $450.00. Hatsan also has a pistol/carbine model. Here is a link to airgun pro they have a list of refurbished Hatsan's for sale.

http://www.airgunproshop.com/product-category/hatsan-refurbished/
i hope this helps. 
 
Flyingdragon air guns can sell you a great shooting pcp airgun and pump for less money than anywhere else. I bought an .177 FDPCP when they were on sale for $80 bucks shipped and I love it as is with its fibre optic sights. It Shoots very well and is easily pumped to 1500 PSI and gives me 40 shots. But if I was only going to buy one airgun it would be my Daystate Huntsman Regal .22. But it was 17 times the price of my FDPCP. But oh what a fantastic beautiful and accurate gun it is!