I still think that this unit lacks any purpose, why would you intentially decrease Civ's setling capabilities like this, where does this cost come from? Is this a cost of a lighthouse and one citizen? I dont think that it worth it at all, especcially when you can buy only one of these at a time. If you made this purchase optional it would be great, otherwise i dont see the point. This feels like a punishment, especially at the very start when you want to settle your first 2 cities, how you gonna do this with Asamu when it takes so much time to build/buy a worker and then build an improvement to get money from the UA. Pray to god of ruins for free coins? That doesnt sound very appealing.
So yeah, the only possibility to save this proposal is to cut mandatory purchase in my opinion. I dont really know what else can be done, make it able to ignore terrain costs and swim faster maybe? Or grant it some capabilities of a Pioneer and Colonist, like having more initial Population in the city. Or you can make it batte unit, so it would not need a guard to settle safely. Well, thats on you.