Alright, firstly we need to solve the issue of not having an unlimited amount of resources, to which i say that refine crafting (at 48% return rate) ontop of salvaging the bow that is crafted if it does not meet the required quality (6.5 planks in return for a small cost) this would leave us with the total loss per attempt at 160k silver (10 planks). So for each attempt it would cost about 160k to attempt a roll at masterpiece via crafting, now, we must consider the use of other crafters with a lower spec then 400/400, this would lower the chances of receiving a masterpiece but increase the amount of crafting you could do per day, thus making the possibility of obtaining 10 Masterpiece bows in a shorter amount of time. i don't know the exact odds on obtaining a masterpiece from a 400/400 crafter, but to me this seems like the cheapest way to attempt to get one without the need of RNG rolling and keeps the cost low per attempt.



Edit: To add, you could sell the focus crafted bows on the market for a profit, but it would mean planks would rise in price due to the increased demand and bows going down in price due to over inflation on the market, which is why i suggest salvaging them (keeps plank price lower and market won't get messed up from the influx of t8 bows)



Also i do need to add that once you do obtain 10 masterpiece bows you would then need to have them enchanted with the use of runes which would add to the cost