I've just made a very tiny, very simple mod to make building units and buildings cost the same as a Standard game speed while playing Marathon.
The default percent value for all that stuff in Marathon is 300, so I just put it to 100. However Settlers still cost 300% of standard when I fire up a marathon length game.
My XML looks like this:
<GameData>
<GameSpeeds>
<Update>
<Set TrainPercent="100" ConstructPercent="100" CreatePercent="100" ResearchPercent="315" />
<Where Type="GAMESPEED_MARATHON" />
</Update>
</GameSpeeds>
</GameData>
The default percent value for all that stuff in Marathon is 300, so I just put it to 100. However Settlers still cost 300% of standard when I fire up a marathon length game.

My XML looks like this:
<GameData>
<GameSpeeds>
<Update>
<Set TrainPercent="100" ConstructPercent="100" CreatePercent="100" ResearchPercent="315" />
<Where Type="GAMESPEED_MARATHON" />
</Update>
</GameSpeeds>
</GameData>