on a few occasions i have actually reached the maximum amount of credits you can have at one time and although i have tested out a few downloaded ships none have come close to using all my money.
however, there was mention in one of the other posts about putting a restriction on the size of a ship. if you limit your size you can in a way limit the cost of a ship. limiting the cost or the size of the ship would make things easier for servers because it would reduce lag.