Effects of Tax Collectors
A tax collector is necessary to collect most of the census tax that a city creates. It also increases the province tax (and thus also the census tax), and the production income of the city by 1 ducat each. On the downside, a city with a tax collector suffers an increase in the revolt risk of 3%.
Promoting Tax Collectors
A bailiff may promoted to a tax collector in any city you own and control, if that city does not already have a tax collector. The base cost for this is 50 ducats; the cost is modified only by inflation. The time required for the promotion is 12 months. During this time you get none of the effects. Note that if you lose control of the province while the promotion is underway, it does not happen and you lose the money.
Removing a Tax Collector
Once you've promoted a tax collector, you cannot voluntarily get rid of him. However, when a province changes ownership, any tax collector in it is removed.