Yea its bloody expensive but mining manually till 300 takes ALOT of time, time you could use farming mines that would in same time sell for more then you would make mining 300 in skill.
Well you only need the initial gold. lets say 100g. On my server that's 4 - 5 stacks of Ore. Selling 4-5 stacks of bars sells for about 80-90g. And btw there are only some stages that are the pain like Silver, silver is a pain in the ass getting as there are no real zone where you find silver. So as long as you can skill the hardest parts with bought ores you should be fine with less then 100g.