Basic:
One well supplies 3 hot fluid per second and consumes 180kW.
The basic heat exchanger recipe converts 1 hot fluid to 10 steam at 180C.
A steam engine consumes 30 steam / sec, at up to 165C, and produces 900kW: exactly the output of one well, ignoring the slight over temperature.
So (with approx 20kW for the exchanger) one well can supply 700kW of power, and is expensive (okay, not too different cost than equivalent solar, but considering the research, cost, and the fact you can only build them where you find geothermal spots, one would expect better).

Advanced:
The well is the same.
The heat exchanger converts 2 hot fluid to 30 steam at 500C.
A steam turbine consumes 60 steam/sec, at up to 500C, and produces 5.82MW: 4/3 the output of one well.
So one well produces approx 4.2MW, factoring in well and exchanger overhead: quite significant at six times the output, but still much less than nuclear (40MW for one reactor, 160MW for two).

The exchanger:
All recipes take 0.5s, but exchanger has speed 4, so can consume 8/s (2.67 wells) with the basic recipe and 16/s (5.33 wells) with the advanced recipe.

The recipes:
The basic version requires 250 R+G+B science, which is 1/4 the cost of nuclear (possibly nuclear is too cheap, but never mind).
The advanced version requires 400 R+G+B+P science, which means it will probably be researched after nuclear.
The buildings both cost a couple of hundred concrete/brick, 10 or 40 advanced circuits, 80 or 40 steel, plus copper and electric engine units — cheaper than nuclear stuff, but not by a whole lot.

Suggestions:
- boost well output to 5/sec; (approx 1300kW basic, 7MW advanced per well)
- possibly also boost steam output in the basic recipe by 50% (approx 2MW basic)
- normalise geothermal fluid consumption in the exchanger to both consume the same per sec, so we don't have excess exchangers when upgrading to advanced
- set the exchanger speed to 1 and increase recipe input/output amounts to get desired throughput
- aim for roughly 3 wells per exchanger; maybe max 18 fluid per sec (9 per 0.5sec recipe, outputting 9*15=135 steam)

Also increase building HP, because they are expensive (e.g. steam engine is 400; I went with 600 and 800).

I just made the above changes to try; at the basic level 3 wells now requires 7.5 engines and produces 7.1 MW - 600kW running power; maybe it's a little OP but so is everything in the game, and at least the power plant now feels like a worth-while investment.

I have slightly increased the basic power production, and added a config for custom multiplication. I also increased well output from 3/s to 5/s, and made the buildings have more health. I will not be changing technology costs, ratios, or per-machine speed.

So your new version produces 12-to-1 steam-to-hot-fluid for the basic version, and 15-to-1 for the advanced version. Fair enough (though the temperature difference is much more significant).

Your exchangers can consume 124=8 and 224=16 fluid with basic and advanced recipes; that difference is enormous and means you now need two exchangers (basic level) for two wells. I still think you should bump the basic exchanger rate, but up to you. I will continue using my version of the mod.