This is an interesting question. If I were to add the price of all the ereaders I've bought and all the ebooks I've bought, then both numbers would be exorbitantly high and I'd be depressed thinking about it! In the end though it probably evens out.
I bought my first ereader in March 2007. In the beginning the reader cost probably exceeded the book cost. I started borrowing books in 2009. The reader and book cost probably equalized. By 2014 I had increased my libraries to three and joined a subscription service. The cost of readers meanwhile was declining. So I transitioned to books costing more. However my annual spend on both readers and books has been decreasing year over year. And, the cost avoidance of the books borrowed probably exceeded both the cost of the reader and the books in more recent years.
2016 will be interesting. I bought an Oasis which at $312 defies the trend of declining reader prices.
So far I have spent $87 on book purchases and my subscription service. The cost avoidance of books that I have borrowed is $306.