If you're an arcane caster who has fireball on your spell list, you don't need to deal with the goggles -- you can get a runestaff (MIC p224) of fireball (costing 3*3*400=3600gp), which does just what you want.
If you don't have fireball on your spell list, the news is worse. D&D tries to avoid giving characters access to spells not on their spell list: wizards can't heal, and clerics can't throw fireballs, and there are ways around this but they're difficult and expensive.
You've found a partial solution to the problem in this goggles item, but as you've noted, it leaves you with a low save DC. I would probably advise your DM that, if your character doesn't have fireball on their spell list, then being unable to cast it effectively is part of your character balance, and your DM should not give you a custom magic item that would solve that for you.
(If nothing else, it's really hard to price something that lets you cast a spell not on your spell list, because it's hard to find something to compare it to!)
If your DM is at peace with the balance issues here, and simply wants to know a price, you could ask them to let you use "wearing a goggles that lets me cast fireball" to satisfy the runestaff's prerequisite of "must have fireball on your spell list". Then you could just use the runestaff. Or you could pay 1.5*3600=5200gp extra to build the runestaff's functionality into your goggles. ("For multiple different abilities in an item, multiply the cheaper cost by 1.5"). The augmented goggles would then cost 9200gp in total.