I believe your equation is incorrect, since it shows 500 and not 700 as the initial cost of the speeders.

It's also possible that the bounty hunters get paid a base rate of 200 per day and 50 per spy, and that the spies are available on the first day. We'd also have to assume that we don't have to pay the daily rent for the first day for the speeders.

Using my interpretation, the formula would be 300t = 700 + 100(t - 1), in which case t = 3. (Edited for an error of my own.)