I made a functioning blackjack table prefab once where I gave a value to the suit, a value to the, uh, value (A, 2, 3.. J, Q, K) and then when the player was assigned cards, used a combination of logic_case and game_text to determine and display the output. The resulting entity setup was pretty impressive to look at, because it was laid out exactly like the flowchart I used to design the logic.
Unrelated? Not quite! My point is, know what entities you have at your disposal, and then draw a flow chart of how the logic will flow, then convert each point of that to a Source entity on your little drawing. It makes complicated ent setups easy!
Basically,
1. Have a logic_case, with case 1 as 0, case 2 as 100, case 3 as 200, and case 4 as 300.
2. Have 4 game_text's, named text0, text100, text200 and text300, with message text like 'Counter: 0' etc. or something
3. For the outputs of the logic_case
a) OnCase1, text0.display
b) OnCase2, text100.display
c) OnCase3, text200.display
d) OnCase4, text300.display
4. The output of your math_counter should then output it's value to the InValue of the logic_case.
Hope that helps!
HOWEVER:
I would suggest, rather than using the UI to display the value, create 4 textures with the values written on, have a counter on the wall, create 4 func_illusionary brush entities textured with the seperate textures all in one place, as part of a counter on the wall. Then set the alpha of the appropriate one to full and the other to 0 via the OnCase1 outputs of the logic case. Would look much more impressive