Adding fertilizer to carbon-rich composting feedstock will help to speed the process and produce a better final product. Blood meal and feather meal are the organic fertilizer sources with the highest levels of nitrogen and would be good choices for this application.  Ideal compost has a ratio of about 30:1 Carbon to Nitrogen.  Fresh horse manure is 25:1 already and composts nicely on its own when hot composted.  For the sawdust, its C:N ratio is from 200-400:1, so you will want to add plenty N fertilizer to encourage rapid decomposition.  In terms of cost, it will depend on how much fertilizer works best for your specific batch of materials and it will likely take some experimentation to determine the right amount to use to make good compost.
Here is more information on composting in general:
http://extension.missouri.edu/p/g6956

Good luck,

Weston