Note that you cannot alter an image's PPI (pixels per inch) in-printer, only DPI (dots per inch). Modifying PPI can only be done in an image editing application. By various terms it's called “up-res'ing” because you're upping the resolution through interpolation. That is, you're making up data that isn't in the original file. This is why you didn't see any appreciable increase in image quality. The data simply wasn't there. Remember you can always throw data away, but can never get it back. Hence the desire for "lossless" compression schemes. As a side note: while often seen in use –and many applications for "print" support their import– .JPG is not a lossless scheme, and is therefore not a print-quality image format in any but the most marginal quality situations.
Considering the ink usage question, think about this from a physical/ mechanical standpoint. If you go from laying down 300dpi to 600 dpi, you’re taking a single dot that defines an area of color (nominally the size of a pin point), and laying down two dots half the size of that single dot in the same area. In order for those two dots to be distinct as two dots, there must be space around them. Therefore, there ought to be fractionally less ink consumption.
Actual results may vary.