The price tag for a brand new in box Classic Army m15a4 with no internal upgrades or external addons would be between $500-600, not including taxes or shipping. Though if you know where to look you might be able to find one for as cheap as $450. You'd have to be Age Verified for that though.
Though CA has pretty good stock internals, if you wanted to upgrade them, that'd probably be in the area of (very roughly) $100. For the externals you see on mine there: $300ish for the m203 grenade launcher, $40ish for the red dot sight, $20-30 for the Z rail, $50 for the taclight. $450-550 for grenades (6 grenades) and $60ish for mags (12 mags).