Just googled for this part...doesn't seem to be any used ones available...and new is over $300

I assume this is what I would need:
ACDelco MAP Sensor - 1998 NISSAN 200SX 1.6L . There seems to be very little information on google about this part and I can't find anyone selling it used. Does it go by any other names besides map and manifold absulute pressure sensor?I don't remember seeing anything that looks like this under my hood but I'll take a look again tomorrow.
Another question relating to fuel economy: when I bought this car it came with what looks like an aftermarket gas cap with a vent in it. Is there any way this could be causing a fuel economy issue as well?
thanks for your help.
Edit: doing further research, I found this on ebay
NISSAN MAXIMA SENTRA 200SX BOOST MAP SENSOR PS54-01:eBay Motors (item 120480444924 end time Oct-23-09 15:13:34 PDT) . Is this the part I need? I can't tell if there are any differences between this and the new one I linked. So this is the part that says Boost on it towards the top-back of the engine bay?