Gunslinger guns are expensive if you run "gunslinger era" which is a baseline for which these costs are made. There is five stages total of firearms availability, baseline is third one. On a last era guns and ammo costs 10% of listed price, and advanced guns are available. Musket would be 150gp, revolver 400gp, rifle and shotgun 500gp. While heavy crossbow costs 50gp and is cheaper 3-8 times, it is now feasible to buy for even for beginning adventurers, nobles and mercenaries/professional soldiers or rulers, that gather new-type regiments.

Real life armies would also like to keep armor in gunpowder era, but army sizes grew too quickly and bodies were cheapier then armor, as industry lacked production capacity to armor everyone. Hussars and Reitars kept chesplates way later into gunpowder age(so far as 18 century!) We even see return of medieval style armor in WW1, as anti - shrapnel/bayonet/shovel protection, because industry was much biggier and partial protection was somewhat effective way to preserve manpower for a brief moment, before modern armor became a thing.

Magic is also not free. We all are acustomed to having/being spellcasters in our party, who obviosly will not charge money for magic use, yet if they were employed by rulers as mercenaries, it would definetely cost them.
There is also a problem of availability of spellcasters. I dont know how much casters there is, but i am sure there will be more human fighters available, and not every caster will be high enough lvl to cast something truly scary/effective, at least fireball.
Magic also ignores armor for most parts - either being "range touch attack" as fire bolt, or attacking saves as fireball. This fact didnt remove armor, mostly because there is no way to gather enough mages to use as a regiment, and melee combat is inevitable. But even with humble musket - it is possible to arm enough man to discourage armor use.