The main cost of the conversion process is the State Conversion Fee, while the costs of the conversion ulpan (preparatory institute for conversion) and the circumcision are subsidized by the State and the Jewish Agency:

Conversion fee:
Beginning in 2005, there is no longer a conversion fee.

Verification of spouse's Jewishness:
If a convert is married to a Jew who is a new immigrant, verification of his spouse's Jewishness will be required, a process which entails a fee of 230 NIS (as of January 2009), paid to the local rabbinical court

A conversion ulpan:
In ulpanim of the Ministry of Education and rabbinate (including those in absorption centers), classes are completely free; in the ulpanim of the Institute for Jewish Studies, a one-time payment of 150 NIS per person or 250 NIS per couple is required, except for soldiers who are not charged; conversion ulpans which take place in kibbutzim and include food and accomodations cost 1,500-1,800 NIS, except in special cases.

Brit milah:
The brit milah which is performed as part of the conversion process is paid for by the Chief Rabbinate, on condition that it is performed after a referral from the bet din, and in a hospital which has an agreement with the rabbinate and performs the brit according to its directives.

A Hebrew language ulpan:
If no conversion ulpan is offered in Israel in a language that the convert knows, he will need a Hebrew ulpan (which is available free for all those eligible under the Law of Return; others will be charged).