The first cart to run commercial DS roms (badly*) was the neoflash** and despite being something of a rebadged XG2 (which was in turn a tweaked EZ2) did cost a pretty penny (more than an equivalent XG/EZ2 and more than those nice carts like the EZ3) , but we already saw NAND-NOR-PSRAM carts like the EZ3 and the start of the memory cards stuff in supercard and GBA movie player as well as the M3 and such. However we already had DS homebrew on just about everything so people were somewhat apprehensive and mainly waited it out. You also have the PS3 USB hacks that cost a fortune at first for what they were (recall what the exploit eventually got ported to).
So yes makers (and resellers) tend to milk it for all it is worth and frankly why not- a serious profit from fools/those with too much disposable income is nicer than scraping one from small margins on limited time devices (either through developer dropping support or devices not able to counter new security) and being left holding the ball (stock of old now useless devices) that current hacking devices seem to be about.
Also has to be said- there are still those that can get it done but the DS flash card scene is pretty cut throat so many teams if they are still around are operating on a skeleton staff and I am not sure if there will be enough people out there to sustain a proper reverse engineering operation at first/really quickly (you could put one together but it would take a couple of weeks). Of course sometimes the teams can release an early beta of their product under another brand name (the most known examples being the likes of supercard with the iplayer and the GBA movie player and M3 team although I am not 100% sure what went there), EZFlash have done a bit of stuff like this, many more teams made R4 clones or used the R4 name to sell their devices and there is the whole story of Borden (the Americans have the story of Johnny Appleseed- this guy was like Johnny Flash cart company founder/designer).
Equally it could be that it requires or uses a hefty device to run (some of the higher end CPLD and FPGA chips are not cheap to buy and get boards built for) which feeds back into the selling your rebadged prototype thing.
Gun to head to get an idea of money. For a device with $30 of parts and assembly the price is usually around $70 to $120 and more pricey chips might reduce the margins a bit (any more than that and you start to leave the "screw it- it is only money" head space).
*even compared to later GBA slot options the roms had to be extensively patched to work. Also caused a nice redump flood but we still do not mention that around jumpman17.
**I actually found a couple the other day and seen as they are more or less marginally reworked EZ2s I wondered if I could turn them into good GBA carts (the EZ2 1 gig being one of those that gets snapped up seconds after they put it up for sale and in the eyes of some are more desirable than an EZ3).