I think realistically you are looking at less than £150 for a basic Brit force (assuming 100points), probably upto £200 for one with decent range of units. Brit infantry are expensive becasuse of the number of FV432 APCs you will end up buying (at least 2 possibly 3 boxes). You can perhaps lessen this using Heli-borne infantry but I have not explores this option.
The tanks from the starter x2 will bulk out the force as the basis and the Helos add cheap
AT or slightly more expensive infantry delivery.
Soviet forces tend to be a bit more "horde" (hey, they're Russians!

) but also get a bit more in the starter box. I read on the
FoW forum that basic/budget forces still run to 15-20 T-72s but I don't have a list for them so not sure.