Talking about Hungary, there were many forms of tax an average serf had to pay in the 15th century. There was the ninth and the tenth, 10-10 percent income tax to be paid to the king and Church, respectively. Then they had to pay through work (robot) to their landlord - few days a year until 1504, then 12 days/year after. There was the census and munera which were basically obligatory gifts in coin and livestock to the landlord. The plot tax to be paid to the king. Subsidium or war tax, could be imposed once a year. Miling tax, because peasents didn't have their own mil. And quite a few other non-regular forms of tax or tax-like obligations like billet or portage. These were all detailed in registers called urbariums.

So the question how much did a county bring in isn't that simple because some of the income went to the ruler, some to the Church and some stayed with the landlord, plus they can't all be exactly quantified. As you guessed most taxes are still paid in goods - there were simply not much money on hand at serfs, and they normally didn't need much of it either. Like today taxes had deadlines and there were designated places to pay them, which were called dézsma-place (dézsma is folk butchering of the world decima). Also to clarify counties weren't owned by barons or nobles - they were administrative units managed by ispáns (comes parochialis in Latin), a baron may have owned all land in a county and so was practically lord of it, laws made it clear he was not the leader of a county. Notably the counies administration was responsible for collecting taxes for the king.

Then there were other sources of income, namely customs and mining (gold - silver and salt- mines), so naturally counties varied one after another in respect of these numbers. I found a pretty precise breakdown of royal income in 1485: of 628 thousand florin, 61% were taxes from serfs, 13% from salt mines, 10% from minting and mining guilds, 8% from customs, 3,5% from towns, the rest is various smaller taxes such as those paid in by Jews or Saxons.

The average serf household has an annual income of 1 to 3 florins. Let's check the numbers! In 1485 the serfs payed in total of 385 thousand florins, At about 300-400 thousand households it comes out to 1 florin per hh. This is in line with the common line I read that households payed in over half their yearly income. With roughly 6-9 thousand household per county, a single county contributed about 6-9 thousand florins to the treasury. So how much did a landlord owning the entire country get? Well he got the ninth which is 10% of a households income, so about 10-30 denarius (100 denarius = 1 florin). Plus the robot but at time that was only a few days a year. The mentionned gifts were comparable in worth to the robot. Sticking to about 20 denarius per household, your county-owning noble enjoyed 1000-2000 florin income a year from his serfs. But keep in mind lot of this was in form of goods not coins, and there were big differences going from coutny to coutny depending on population size, economic profile and activity. Also Hungary had like 90 counties at the time, so someone owning a county wasn't necessary a very relevant person.

Sources I used:

Hungarian Ethnographic Lexicon

History of money in Hungary, Márton Gyöngyössy

Financial and economic plicy in the Kingdom of Hungary, Bálint Hómann

Life of Serfs, György Domanovszky