When a peasant woman married, there were either one or two notable expenses besides the wedding itself: the dowry and the merchet. This second item was a fee paid to the lord on the marriage of an unfree peasant woman, essentially intended to counter her loss as a worker and some land as dowry. How much the merchet actually was depended on the lord, his bailiff, the woman’s own property, and the size of her dowry. Non-payment, no matter the size, could result in the seizure of all of her family’s property.
The thing is, merchet applied only to two groups of peasants: unfree peasants (i.e. serfs) and peasants who held land that was designated unfree but were themselves free. In other words, the obligations of work and taxes that were required of serfs could also be tied to the land itself instead of or as well as to the people who worked it.* So technically the lord or his bailiff could demand merchet from a free peasant woman, provided her family held unfree land. How often that happened though is unknown.
It’s also worth noting, though, that not all unfree peasants paid the merchet either. Before the 13th century (and perhaps even after in more out-of-the-way places) there was the question of clandestine marriage. Some people might be declared married and required to pay the merchet, but others might simply live together as married without anything official. Also, as time went by some families managed to conceal their serf status and get out a lot of their obligations.**
Then there was the question of dowry and who the woman married. The amount owed for merchet often depended on the size of the dowry. A woman with a dowry small enough might not have to pay anything. If she married someone under the lord’s jurisdiction, the merchet would be smaller. If she married farther afield, though, the payment was correspondingly larger.
A lot depended on circumstance and location. The amount owed varied depending on who the woman married, where she lived, and what her family owned. Some families managed to hide the fact that they owed merchet entirely, either by obscuring their serf status or obscuring whether or not there was actually a marriage.
*There are a number of ways a free peasant could come to hold unfree land. To name a few, they could inherit it from a relative who was a serf, or even from a free relative who already held unfree land. Or they could buy it from another free peasant.
**In some cases it became a secret they were willing to kill to protect.
Payment of Merchet, Pope Gregory the Great - Internet History Sourcebooks Project
Searle, Eleanor. "Seigneruial Control of Women's Marriage: The Antecedents and Function of Merchet in England." Past and Present82 (1979): 3-43.
Gies, Frances and Joseph Gies. Marriage and Family i the Middle Ages. New York: Harper & Row, 1987.