According to the SRD a weapon with a weapon enhancement bonus of +6 is an epic weapon and has a market price of 720,000 gp. Supposing a non epic special ability is added to the weapon, say for example flaming which has a +1 price bonus. Would the total market price not including weapon cost and masterwork quality be 722,000 gp or would it be 980,000 gp as per an epic weapon with a +7 enhancement bonus equivalence?
To obtain the price of any enchanted weapon or armor, you add the actual bonus to the bonus modifier of any enchantments, and then use the total to look up the price on the chart. So yes, a +6 weapon with the flaming enchantment would be priced as a +7 weapon at 980,000gp. This might seem excessive, as flaming is not that significant a modifier at epic levels, and for this reason a wizard might be reluctant to bother putting a flaming enchantment on a +6 weapon, but that is how prices work in 3rd edition.
On the other hand, at epic levels, the amount of gold you can be expected to have acquired and have access to acquiring is monumental. There comes a certain point in a campaign where money often starts to seem like it is an unlimited resource... and if it doesn't, players may very well feel extremely underpowered because of it. A glance at various high level supplements will show you that both prices and quest rewards can start getting pretty ridiculous, which is why astral diamonds were invented as a currency.
A DM is of course free to houserule this, but I might advise against it. If you are playing at epic levels, it's probably a lot more reasonable to just lean into the insanity. Yes, this sword costs a veritable mountain of gold (or the equivalent in exotic materials) to enchant. That's just how you roll at this kind of power level. If you want to follow in Karsus' footsteps, you're going to need to bankroll it.