What about ion engines? I'm extremely not sure I have this math right but NEXT claims 4190 second ISP [0]. Plugging that into the rocket equation I get:
iss_mass = 419600
sep_isp = 4190
delta_v = 5748
htv_payload = 3310
falcon_launch_cost = 62000000
total_mass = iss_mass * math.exp(delta_v / (9.8 * sep_isp))
fuel_load = total_mass - iss_mass
dragon_launches = math.floor(fuel_load / htv_payload)
total_cost = dragon_launches * falcon_launch_cost
I get:
Total vehicle mass: 482,646.89 kg
Fuel to add: 63,046.89 kg
Dragon 2 launches: 19
Total cost: $1,178,000,000.00
Not what you'd call cheap but at least in the realm of the possible. I'm assuming the ISS' solar panels (~120 kW [1]) is sufficient to power the thruster, that a dragon launch costs no more than a falcon launch (certainly false), and that the ion engine is already on board the station.
[0] https://en.wikipedia.org/wiki/NEXT_(ion_thruster)
[1] https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station
