I have a 2001 VW Jetta 1.8T and it keeps throwing the PO171 code for "system too lean bank 1"?

ive already given the vehicle a complete tune-up it has low mileage for a ten yr old car but it needed to be done. i replaced the vacuum lines as well so im thinking its the MAF sensor. any thoughts? and my engine code is the AWW model which is the more expensive sensor, where can i get the cheapest one that's not used?