Farm upgrades are worth ~20 wood per farm.

That is ~20 wood per farm that you build upgraded that the enemy builds un-upgraded. I made this post because other explanations frankly are too hard to turn into a useful heuristic. 20 wood per farm is simple, relatively accurate, and helps decide trade-offs between eco-bonuses.

The following assumes farmers gather at a net rate of 20, 22, then 24f/min respectively. However in general the wood savings is not really affected by the farming rate because it affects both the upgraded and un-upgraded farms. All numbers are approximations obviously.

1. Horse-collar: For the un-upgraded farm you need to gather wood at a rate of 6.857 for 8.75 min to afford the next farm. For the upgraded farm you need to gather 4.8w/min for 12.5min to afford the next farm. (6.857-4.8)*8.75 = 18 wood saved per farm. Assumes un-upgraded farm gets upgraded upon reseed.
2. Heavy Plow: For the un-upgraded farm you need to gather wood at a rate of 5.33 for 11.25 min to afford the next farm. For the upgraded farm you need to gather 3.52w/min for 17min to afford the next farm. (5.33-3.52)*11.25 = 20.4 wood saved per farm. Assumes un-upgraded farm gets upgraded upon reseed.
3. Crop Rotation: For the un-upgraded farm you need to gather wood at a rate of 3.87 for 15.5 min to afford the next farm. For the upgraded farm you need to gather 2.6w/min for 23min to afford the next farm. (3.87-2.6)*15.5 = 19.7 wood saved per farm. Assumes un-upgraded farm gets upgraded upon reseed.
   1. If the enemy does not upgrade their farm upon reseed you get that wood savings again. So for crop rotation, if the enemy doesn't have the tech, you get 19.5 wood saved for each time the enemy reseeds a farm. However that's still only 1.27w/min per farm because of the slow reseed rate.

Also as a rough heuristic heavy plow nets you an extra 1 food per farm per minute. So if you have 24 farms and get it 5min before your opponent that's 120 food.

As a comparison double-bit axe gets you ~3 wood per lumberjack per minute. So every minute earlier you research it is worth exactly that. For 10 lumberjacks that's 30 wood every minute. Put another way: Building 3 farms without horse-collar is equivalent to delaying double-bit axe 2 minutes w/ 10 lumber jacks. Obviously you need to estimate a discount factor (because strictly speaking the lumber benefit is delayed until the enemy farm reseeds), but even if you claim resources halve in value every ~4.5 minutes that implies waiting 30 seconds is equivalent.