Parable books has the Adobe Digital Editions EPUB version of the NIV Study Bible for $8.99. Other stores (Amazon, Kobo and Christianbook) have it listed for $25.99.
Here is the link at Parable Books. Note that though the product page says that it is the Red Letter Edition, the ebook that was delivered is the regular "black letter" edition. It matches the ISBN on the product page, however. Also note that the file is relatively large (~50MB) and some people seem to experience problems downloading such large files with Digital Editions.