[OXO cube]

below are my tyre truers 
all perform a different roll 
but essentially do the same job
     
from left to right :-
3DP  hudy 'type' tyre truer , file freely available on 'thingiverse' , works great and a fraction of the cost ...... off axle single tyre truing 
my own design  'proses' type 3DP tyre truer plate , used mainly these days to re-skim already trued tyres , or truing smaller scales (1/43,1/64)..... on car tyre truing 
Area 3 tyre truer , ( similar design to RSM & Razor ) , as can be seen is my 'go to' atm ..... on axle both wheel tyre truing ( tbh i only true one at a time , then turn the axle over to true the other side , but thats just me )

realistically , this hobby can become expensive depending how far down the rabbit hole you go , but then again it is all personal preference 
as Andy states a sanding plate of any description 'IS' a basic truer as described in the title , and the proses one is probably the cheapest factory produced item out there
heck my 1st ( and i bet alot of others ) was sand paper taped to the track , with patience can give good results and was a great introduction to the 'mystery'

luckily i have access to a printer which helps  , if anyone wants the STL file for the truer plate , just PM me 

Kev

( ducking already   )