puhh, a long time since i played EU1.

Do you hired Cav at this province allready?

Then my only two ideas are
I.The price shown is only the baseprice, Inflation and Settings could raise the final price so high that you can't afford recruiting cav., but with 257 gulden left..it would be a insane rate^^

2.There was a recruitment glitch in EU1 and 2 but it should be patched long ago, restarting/reloading solved it.