We've been using a 3-in-one pH, soil moisture, and light meter and while it's pretty approxamative it works well for us. It cost 14 euros and does the job.
We've recently ran into a problem in which our growing medium, which was once about 3 or 4 cm thick is now around 1 cm. To make matters worse, the meter has two prongs and can't fit into the container holes in the bottom.
We've found a meter more or less like ours with one prong, costing 40 euros. That could work.