You probsbly can afford to buy it but you need to upgrade it right away for it's full benefits I doubt you have more than 1 bil
Looks to me that if you have max plunder and convert a level 3 forge to a level 2 sub at a cost of $480m plunder goes up by 31k. If you go straight to level 3 sub, then plunder increases by 50k over a level 3 forge at cost of $1.33b