It's not correct to say cost is a factor. A £100 GPS will give you true course so all navigation could be expressed in true. You might still have a magnetic compass in a Cessna 150 to give you a rough idea which direction you are pointing when stationery and you could use if it you want to fly a track manually. If your chosen track was 137 true and you were pointing 130 magnetic but tracking 133 true, you could guess holding 134 magnetic might do the trick. You don't actually need to know what magnetic direction you are heading - it could be in radians. The compass would just be a convenient datum to help you achieve your true track as shown by the GPS.