in pistols, they pretty much start around $120 (most that are less tend to have spotty quality, or have poor performance, e.g., low muzzle velocity or highly inconsistent accuracy.)
Most from the major producers - Tokyo Marui, Western Arms, KSC/KWA in the US; there are some other very good brands like Marushin and Tanaka Works that are not widely available in the US - are pretty impressive.
The "springers" and lower end ($10-$90) gas guns are pretty consistently "toy like" and "lob" their bbs.
The rifles and SMG's - either gas or electric "AEG" - are also pretty darned impressive. Classic Army; Tokyo Marui are at the top of the heap in the US (there are other high end brands outside the US); Echo 1, ICS, others are (close) seconds.
These run $200 to $450.
There is also a very high end parts house turned complete gun brand that has created a whole new standard - Systema out of Japan.
The AEG's have pretty much driven the gas rifles and SMGs into extinction, but there is a thriving niche market that looks at old time gas rifles made by mostly extinct companies like JAC, MGC, Asahi, et al as the pinnacle of both realism and performance. "Classic airsoft" tend to shoot hot (400-600 FPS) and tend to be very realistic, including brand and proof marks.
The older gas pistols were highly realistic in their feel and weight, though their gaskets and valve designs are obsolete and not quite as reliable and consistent as the post Magna blowback period (introduced by Western Arms)
MGC HK P7 (shown) and Tanaka HK USP heavy weight; MGC Glocks heavyweight, et al are from that period, and man, are they realistic in feel! I wouldn't use them if my skirmishing life depended on them, though.
Cheers,
TM