The document, attached on the above page, is an excellent general guide to running a manual drilling business and was published by UNICEF (the original can be found at on the UNICEF page). Module 4 in the document together with the 'Cost Price Worksheet' in Annex 2 are especially useful for working out how much drilling a borehole costs and therefore how much you should budget.
Another very good document, published by RWSN here, gives lots of useful notes on what to budget for and roughly how much, it is aimed at drillers using large engine powered rigs but it still has a lot of useful information for a smaller manual drilling businesses too.