.177 is the caliber (i.e., 4.5mm)
airsoft is 6mm
Typical .177cal BBs are 5.1 grain (~0.33grams)
Airsoft BBs typically range from 0.2grams to 0.3 grams
You won't get anything worth two shits for $150 in airsoft. You can get a decent plinker airgun that'll shoot straight and last a decade for $150.