I'm pretty sure someone in Nation gave a chart damage. According to that chart, you're better off getting even battle swords.
Im fairly certain both battle swords would need to be R6 and have ridiculous enchants like 56th floor and such, which would be wat im trying to point out about tons of money. Special upgrading 2 swords would be extremely expensive over enchanted focused beams. And if im going to go all out and special upgrade 2 swords I might as well just wait for royal celtic swords and do it on them. If you could show me the chart I'll see where the break even point is and figure out wats worth and wats not.