Hmm, I haven't seen a whole lot about the costs of creating cursed magic items. I suppose the game makers assumed most PCs wouldn't be in that business much.
Given that the PCs are only 7th-ish level, I'd probably just start with an existing item of the appropriate level, and keep it as is. Then add on that the costs of adding
bestow curse, and give it some curse effect in line with the effect of that spell. So the resulting item is cursed in some way, but also beneficial in its normal way.
Then I'd raise or lower the curse cost in 25% chunks, depending upon how you power up the curse, how/if it interferes with the item's normal function, whether it allows a save, how easy it is to negate the curse or ditch the item, etc.
But really this stuff is just guesswork. Using this method, my quick price estimates of a few items I can think of off the cuff are popping out market prices (ie, commissioning prices) of 15000gp - 30000gp and more, even for minor Wondrous Items. So yeah, even DIY, the creation gp costs for some of these things are 5x or 10x the price of the uncursed item!
I can post some ideas if you're interested, but you might ask the DM for better guidance on pricing & balancing curses, since this could lead to an arms race if not done carefully.
