It depends on what you call cheap its 200-250 to get a bottle and guages etc then you can get hundreds of fills from that one bottle every time the bottle is filled which only costs about a fiver or you could get a pump for 90 and spent an hour pumping each time you fill
Feck me , i wont be visiting your local feild sports shop
unless the seals are old in youre gun the preasure should stay the same last year when i had my gun for christmas i pumped it up in november and then left it untill christmas day and the preassure was still the same