I think it would be best as a one-time payment. You pay to reduce the costs of a certain number of soldiers and once that number is up, it goes back to normal costs. Of course, we would then have to start tracking number of soldiers recruited from the center and reducing the counter of subsidized soldiers.
For example, Lord Kepler of Keplerville decided that the Kepler Guards, a 200-man center costing 45 gold per ten men, should cost 40 gold per ten men for the next 600 soldiers. So he pays 300 gold now and the a counter is established tracking that next 600 soldiers recruited from that center. When the counter hits 0, price goes back to 45 gold per ten men. If he had wanted to reduce the cost to 10 gold per ten men for 600 soldiers, it would've cost 2,100 gold. Perhaps there should be an 'administration fee' extra cost for subsidizing unit costs.
Fair enough, but it becomes two distinct mechanics, one of them is to subsidize with set amounts upfront, the other is to tax per transaction as they come.
