What is the cheapest and easiest way to add a power meter to my Diamondback Haanjo 4?

I was going to say that you should send the left crank arm into 4iiii to install the strain gauges. They charge US $300.

However, I checked, and they do not list the Alba as eligible for this service. The reason likely has to do with economics. All power meters use strain gauges to determine how much you are flexing the crank arm (it’s more complicated than that because the gauges need to isolate the amount of flex in one dimension, but bear with me). Beyond that, however, a power meter company needs to determine how much the crank deflects under a set amount of torque. All cranksets have different wall thicknesses and materials, so all will deflect a different amount for the same amount of force. In theory, I see no reason the Albas can't take strain gauges. However, they're entry-level cranks. If you could convince 4iiii or Stages to do the calibrations, you could get either company to do the factory install, but they are likely to judge that the demand for Albas with power meters is too small.

Shimano 105 through Dura Ace cranks are all very common, so it’s clearly worth it for power meter manufacturers to validate their meters on all those cranksets. Campagnolo is a much less frequent groupset choice, so companies may only support some models, e.g. 4iiii only supports Potenza, Record, and Super Record, leaving Chorus, Centaur, and everything below out.

Praxis is a much smaller brand. 4iiii supports the carbon Zayante cranks, but not the Albas. Stages doesn’t support them at all. Pioneer, while they sold power meters, only supported Shimano cranks.

There are some options. I believe that some versions of the Alba have removable spiders, and Power2Max should make a spider-based power meter compatible with this crankset. You could buy a Zayante crankset with a power meter pre installed. Especially if you already use Look road pedals, you could buy pedal-based power meters instead; of the available ones, the Favero Assioma appears the cheapest and it is also very well-regraded. All these come in cheaper single sided variants as well as dual sided.

Sadly, you will need to change your bottom bracket to use Shimano cranks or a BB30 crank with power to your bike. The M30 Bottom bracket has a 30mm spindle that tapers to 28mm on the non-drive side, whereas BB30 is 30mm throughout, and Shimano is 24mm throughout. Changing the bottom bracket would make switching cranks more expensive.