Actually it's not that 35 thousand is idiomatically "singular", but that in this context it's a number. (More formally, a cardinal number.) So 35 thousand is the number, the unit of currency is the thing that is plural. 35000 dollars. 35000 euros. For the same reason you would never say it costs tens. Or twelves. Or twenty-ones. It costs ten (dollars), twelve (euros), twenty-one (francs), etc.