Hm, maybe one of the full size Ducky boards? I forget the model number...maybe 9008? You can try your luck with Rosewill RK-9000s as well. I've heard varying reports of them being fine or going bad quickly. If you can stomach how they look, the CoolerMaster Storm Trigger might be nice for your price range. And finally, its a bit more but I type on a Filco Majestouch 2 at work.
Now there's 2 catches. First? If you're looking for ISO layouts, I'm not sure if all the boards I listed have that option. I'm an ANSI user myself. Second? I'm pretty sure there isn't a daughterboard chip/firmware swap that you can do to make these boards programmable. The only chip I know of is the HID Liberation device but it only fits a Filco TenKeyLess board. However, people always recommend AutoHotKey to remap the layout. I'm not 100% as to how the HID device works so you'll want to do more research on that.
Good luck with your hunt!