I mean....yes the guns are expensive and in many cases the gear is too but one of the things that sets airsoft aside from paintball is the cost of ammunition. A typical bottle of 3000 rounds costs what, a tenner? So 0.33p per round. So let's say you give away a hi cap's worth, we're only talking about a quid. A mk5, £2? Maybe £2.50 if you've bought it onsite. If these tiny little amounts are that big a deal to you then maybe you've chosen the wrong hobby...
Think about it this way - some rental kid who has clearly been having a load of fun (on account of having run out of ammo) asks you if you have any spare. You look in your bag and yes, you have more than you actually need. You could just say no. He'll go away deflated and you'll have either a sense of pride in having managed to keep that extra pounds worth of ammo for another day OR maybe you'll get that niggling little doubt in the back of your mind that actually, ACTUALLY you could have been a better person back there.
Alternatively you could actually say "yeah sure, fill your boots" (hopefully he wouldn't really fill his boots cos that's WAY more than a hi caps worth). He goes away happy, you get a warm sense of pride that you've improved someone else's day and maybe, just maybe you've made a new friend. Is your life so full and rich that you don't need any more friends? I seriously doubt it.
Life is short. Don't squander it being mean.