The reason you see a price lower than the amount you paid in is that we calculate your crate value on the current market price of the ammo in your crate. As prices drop in the market, the value of your crate will drop, but the number of rounds will stay the same.
If you buy 100 rounds at $1.00 per round, your total crate value will be $100. If the price per round then drops to $0.90, your crate will be calculated using the $.90 so the $100 you paid now looks like $90 in the system. You still have the same number of rounds and future buys will go in at the lower rate.
Said another way using a more common example. If you buy gas and fill up your tank with 20 gallons at $4 per gallon, the "value" of the gas in your tank is $80. If you stop driving for a month and the price of gas drops to $3 per gallon, the "value" of the gas in your tank is now $60. The amount of gas in your car remains the same.