Why is the monero i buy not worth the money i spended on it?

i have bought 1000 ars worth of monero, but when i translate it into monero its worth 300 ars, not even half the money i spended on it

The prices are set by the individual traders. The rate you got was particularly bad, it seems. You should try using a different seller next time, and make sure the price is to your liking.