the issue with cheap, is what whilst it is possible, assuming your new to this there's going to be some avenues it's not worth risking.
the cheapest, "upgrade" is to get into the habit of cleaning your barrel, at least before every game day but bringing a cleaning rod with you is never a bad call. no need to get fancy, a clean strip of microfibre on the cheap plastic rods many guns come with is perfectly good enough. it's pretty much free and there is no real downside.
second upgrade isn't necessarily cheap, but it can at least be tailored to your pocket is to look at heavier/better quality ammo, how heavy is a balance between what the gun can lift, how heavy your trigger finger and how much you can stomach spending.
other cheap upgrades include the aforementioned bucking (in the name of lifting heavier ammo) and going after air seal, where the parts aren't strictly expensive (eg an o ring over a whole piston head) but dissassembling an aeg box is a clockwork nightmare and not the best idea to learn how to do it on your only pew, if you make a mistake it can mean the walk of shame and i can assure you it gets real old real fast.