hvac booster fans

My house is a 100+ year old 2 story on a full basement. The furnace is a high efficiency gas unit that is 11 years old. We took a small bedroom/closet upstairs and converted into a master bath. This room didn't have a heat vent, so our plumber/HVAC man ran a duct upstairs (through the balloon frame; exterior wall). He connected it to a vent that was originally in the basement. No issues with any of that so far. The problem is, there is BARELY any air flow coming out of the vent. I asked him to check into a booster fan to go in the vent. The local plumbing supply house said they could get one for $270 (7" round). I looked online and found some at home depot and other places for around $25 on up to the hundreds. I know very little about HVAC. What is the difference between the lower priced and higher prices ones? We don't have the kind of money to throw that much into just one vent. Is it possible to just buy a more powerful squirrel cage for that furnace/AC? Two of the upstairs vents get really good airflow, while the other two don't get much air. I believe I can diy a booster fan and let him hook up the electric; but a bigger squirrel cage might be a better option to get better flow throughout the house if that if possible. We only plan to be here maybe 5 years, give-or-take, so we don't want to invest in a new air unit or rerouting the ductwork. Ideas, please? Thanks!