Sounds like a bug which should be reported. When a 64-bit unsigned integer underflows (i.e. becomes less than zero, which can't happen in an unsigned integer) it rolls round to 18,446,744,073,709,551,615. It would seem that your rebuy is 18,446,744,073,709,549,561, which indicates a rebuy cost of -2,054 Cr. and a ship cost of -41,080 Cr. at a 5% insurance rate.