Well, AFAIK the UK is paying 2.5billion USD; IF the total development budget ends up at 40 billion then that's 6.251%. However if the budget increases above and beyond that (say, to 55 billion) then we're talking less than 5% of the total development budget.Tiny bit?
Look up the proportion of development money the UK has stumped up and the proportion of the planned total that they are expected to buy, still sound like a tiny amount?
The expression "tiny bit" was perhaps not the best in this context however in percentage it seems not very much.