I have bought a bunch of cheap digital hygrometers from Amazon. They come pre-calibrated and with a defined tolerance which is OK for me.
And they agree with each other. If I put them in the humidor close to each other, they read the same.
I humidity my main humidor with Boveda 72 packs. In summer when it is very humid, they all read 71 or 72. In winter, when my house is very dry, depending where they are placed in the humidor, they would read 64-69. They are working well for me.