Need help boosting home wifi strength...any affordable solutions?

I am pretty average in terms of technical knowledge about stuff like this so I need some help.

Are there any affordable ways to boost the wifi strength in my house? I have U-Verse with their top of the line internet speeds, but I have a 3-story townhome and it really gets slow/loses connectivity on the bottom floor. Due to setup, we can’t put the modem anywhere but the top floor (We rent and it’s a long story).

Anyways, I work from home once or twice a week and really would love to boost the wifi strength. The router we have now was supplied by AT&T and I suspect it sucks. However the TV boxes are wireless as well and I worry changing the router or modem would frick those up.

By affordable, I mean $100ish and something I can bring with me when we move. Do I just buy a new wireless router? Is there a signal amplifier or something I can add on or place on a lower floor?