you prolly better asking this in 'Single & Fixie' forum...
.but.
$150 bike or $1K bike - are you sure of the chainline ? Wheels ? alignment ?
rr wheel might not be properly dished ?
rr wheel cog might not be set to proper chainline ?
Frame is fudged and the dropout alignment is screwed ?
How would you know ?
A quick check which can start the diagnosis... bike on bike stand - visually center the rear wheel between the chainstays....
Assuming the rear wheel axle is straight - measure both sides distance center of crank/BB to forward or back edge of the axle
each side should be same - if a measurement is off, by more than 1mm - it's one of 3 above...
easiest to determine is to have wheel checked with a dish tool...
and/or mount wheel with axle each side spaced equal distance from center crank/BB and check 'chainline'
Sheldon Brown (bless his soul...) as usual, has good intell...
https://www.sheldonbrown.com/singlespeed.html
I prefer using a straight Alu bar to check chainline vs my dubious visual sightline...
...could eventually mean 'the frame'... but go there last...
... wondering why the bike was $150 ?
Ride On
Yuri
EDIT - and obviously check the chainring for 'faults' like a wobble or bent/munched teeth, single speed cog Teeth and extremely wobbly freewheeling? Is the BB/axle properly adjusted so there's no woblle in the driveside crank ?