Generally they don't unless their doing something illegal. Most politicians are reasonably to very wealthy going into office, and being a high ranking politician basically secures them a high paying position in the private sector afterwards. Many people from the FCC/FDA will go into the telecom/pharmaceutical industries for example. Others will charge huge fees for speaking engagements and appearances. I remember reading Bill Clinton charges in the 10k-50k range for speaches but not sure on the specifics.

So basically no one expects to make money from their paycheck, but rather go into politics for the fame, connections, and other intangibles.