Pontiac Repair: 96 grand prix P0401 code, vacuum leaks, p0401 code

Question
I bought a 96 grand prix about a month ago. when I bought the car the SES light was on. I had a code reader hooked up to it and it popped out a P0401 insufficient EGR flow. I changed the EGR valve and it still came on. I had the intake cleaned with a 3 part cleaning system and got a new EGR valve and put it on and it is still popping out the code. I went ahead and bought a cheap code reader to keep clearing the code out myself, and today it popped out a P0401 and a P1406. I erased the codes and the only one to pop back up was the P0401. I'm lost, it's not idling rough at all, it's got great power, it hesitates just a little bit, but when I clear out the codes my gas mileage is excellent, around 29 mpg. The guy who I bought it off of said it might need a tune up and that he hadn't replaced the wires in a while but would that pop out the P0401? Please help me I love the car but I really want to get it fixed before I do any major damage. I've read alot about possible vacuum leaks in the intake. If that's the case how much would that cost, estimated, so the garage won't try to rip me off. Any ideas would be greatly appreciated. Thanks

Answer
There are two things I would check into Johnathan. First is the EGR control solenoid. This is the device that actually applies vacuum to the valve so if it or the hoses are damaged then the valve won't open and you will get a code. Second is where the valve bolts into the manifold if there is any carbon buildup restricting flow or gasket in the way or anything like that, then that would give you an insufficient flow. As far as the vacuum leak from the intake you can check this by spraying carb cleaner CAREFULLY aroung the intake. Any changes in engine rpm would indicate a leak. As far as an estimate goes I would be surprised at one much higher than $500 and even that would be an involved one. Hope this helps you some and good luck :)