Your experiences about a lack of quantity coin-wise are right on. Though I'm not sure from how you worded it whether that coin total for an average per farm, or a total for all 12. If that was a total for all 12 and you spent days and days at each site, then you've earned your masochist merit badge. It's also not totally clear whether you mean the yard around a farmhouse, or a field itself.
In my experience the yard around an old farmhouse should produce something decent, but not as much as other old houses in a more downtown area. Farm fields don't usually produce a lot unless they were heavily travelled or had other events there.
The finds-per-day are usually pretty poor because of the type of land use and the large area involved. If you want to find something, your coil has to go over it, so gridding out 10 acres is going to take a long time if you don't want to miss too much.
That being said, they're still a good way to find older items if you're willing to put in the time, and the digging is easy if they've been plowed so it's not a bad way to spend a few hours relaxing on a nice afternoon or evening.
When I first got back into the hobby in 2005 and bought a DFX, I got permission to hunt a couple of fields that were probably 8 or 10 acres each. Probably spent 3 hours every other day for a solid month on each one, digging everything that didn't sound like iron.
Well, let's back up a second, I actually started the first field with my old garrett and found a 1776 spanish silver 1/2 reale on maybe the second plow-row I did. (THEN I went out and bought a DFX)
After spending basically 2 months solid on those couple fields, I ended up with maybe 6 or 7 indians, 10 wheats, 1 gold ring, 2 large cents, 1 king george copper, 1 or 2 v nickels and a buffalo or two, and two silver coins total. One was the spanish reale, the other was a silver washington quarter. 1 silver from each field. And 90 percent of the coins came from the second field. A fair number of musketballs and half a dozen buttons were also recovered.
So it is a good way to find older coins, but usually you won't find many of them.
There are exceptions where I've heard of lots of old coppers coming out of a field, but that's more the exception than the rule.
Whether it's worth it or not is up to you.