Flash Drive Shield

I am working on some mcu based devices for reading/writing files on USB flash drives. Currently I do not have much time to spend on arduino projects but my friend ask me if I could modify one of my devices so his arduino board could read and write files from pendrives via SPI. Since I do not have much free time, so I was wondering, if I am going to do this I would like to create the whole shield for this with libraries for arduino IDE, so more people could make use of my work. That is why I would like to ask on this forum if you could really use this type of shield. In other words is there a need to create this shield if you already have the ability to use SD cards in the easy way. I realize that there is already a shield with USB host chip, but I was thinking more of a shield that does all the work by itself, so you could interface it via SPI or UART with commands to only manage files, like in dos or linux systems, for example, cd, dir, delete, etc. Another question I would like to ask is how much money would you be willing to spend on this type of shield. I know that the less the better, but I have to choose components for the shield and I have to have some kind of max cost limit. What other functionality would you expect from this shield besides reading, writing files/directories? Maybe MP3 files decoding and synchronously sending decoded music samples to arduino? And one last thing, because it will also have the influence on component choosing (of-the-shelf availability of chips, smd or non-smd chips), how would you like to see the distribution and availability when the shield is finished? Open source/hardware DIY, kickstarter project pledge and then sending finished board via post, ordering online in some shop like SparkFun? I am all ears.