If you use too little toner it will exhaust quickly and it will be uneconomical to use like this. Try to get a good number of prints in a batch for toning and, if possible, have some duplicates or wasters to experiment with. I don't use less than a litre of solution for these reasons (although using 500ml for a handful of 7x5's would be ok). Sepia can be stored after use and replenished, which can be a good way of getting subtle effects once you have become familiar with different ratios of bleach and toner. Tim Rudman and Lee Frost spring to mind for good book sources on toning.
I second the previous Mark on MGIV's toning properties as there are more responsive papers available for toning. See many other threads for more information.
Regards, Mark Walker.
Don't forget that toning is a process for discovery, so don't be afraid of doing it your way if you like the results.