I think the price depends on which country you are in. In Britain, I pay 7.50pounds per month for 200 minutes of calls, texts and 500MB of data. But there are plans where you just pay for usage as long as you buy something every few months.
But to use a smartphone for GPS you do not need a cellular service at all. Indeed, if I am not expecting calls I switch to airplane mode when on the trail and this prolongs the battery life. You can load the maps from a PC via USB or wifi and the GPS/GLONASS signals are not part of the cellular spectrum. In clear conditions I get 5metres accuracy or better.
An alternative (but heavier to carry) is a tablet with GPS and no cellular function at all. This has zero recurring cost.
But to use a smartphone for GPS you do not need a cellular service at all. Indeed, if I am not expecting calls I switch to airplane mode when on the trail and this prolongs the battery life. You can load the maps from a PC via USB or wifi and the GPS/GLONASS signals are not part of the cellular spectrum. In clear conditions I get 5metres accuracy or better.
An alternative (but heavier to carry) is a tablet with GPS and no cellular function at all. This has zero recurring cost.