Main line of thinking was that in previous versions everything gets multiplied by X depending on the mod/time frame you selected whereas with an insta-improvement its a little different.
No, it's not different. The formula used is the same, it's just that 0 * 300% stays 0.
That's totally in line with other mechanics. The one thing that workers from using up all of their charges literally instantaneous is that working on an improvement during a turn takes up the rest of your movement, so the actual number of turns required is of course 1, not 0.
That however, is once again perfectly in line with how the rest of Marathon works - abilities that "take up the rest of your turn" still just take up the rest of your turn, the number of turns is not multiplied with the Speed Modifier to make them wait for 3 turns on Marathon.
So overall, mechanically it fits perfectly into the rest of the Marathon Game Speed.
In terms of balance it's okay as well. Yes, you can put down 3 Improvements in almost no time, but that's it for the worker, it's gone. To get more Improvements you need to build a new Worker, which will once again takes the full 300% Production-Cost of Marathon. There's the scaling, in the production of new workers.
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About that workercost-discussion:
I don't see why they'd need to cost more than they cost in Civ 5. They no longer provide "infinite" value but are instead used up pretty quickly. In the preview footage build menu they now cost half of what Settlers cost, that may of course change in the future, but it gives us an idea where the devs want the cost to be.
And I think the costs are reasonable. In some ways you can think of workers as "buildings that are not unique but instead limited by your population and terrain." - If you use a worker to build 3 Farms then that worker basically produced a "+3 Food Building". That analogy does of course not work entirely, as you can build workers in other cities, have to defend all of those tiles, etc. - but it should give a hint about why the worker cost don't need to be pushed into extraordinary terrain, they're limited value.
And of course the devs probably want to encourage building workers for as many tiles as possible, because Improvements make your empire look good and provide decisions that need to be made. So overall, workers should be rather cheap.