If its just code retrieval your looking for, then a cheap Actron pocket scanner will do just fine. About $80 or so new?
If your into codes and watching what your engine is doing by viewing live data, then your looking at about $200 for a full fledge scanner.
Then you have the scanners that plug directly into your laptop. Unlimited data storage, bigger screen, you can take snapshots, record data etc. Like the one I have from Scantool.net.
If your interested in reprogramming like Billboost37 and Danthurs has(HP Tuners) those things do everything like above, but also reprogram. Those little boogers run about $400-500.
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8