With some hackery you can get an RTKLIB compatible GPS for as low as $30, but one should expect to pay no less than $200-$400 for a ready made solution with an

With some hackery you can get an RTKLIB compatible GPS for as low as $30, but one should expect to pay no less than $200-$400 for a ready made solution with an useable antenna. Costs go up fast from there.