There is an oft-quoted result from Google stating that when they switched Gmail to site-wide SSL, they saw only a minor overhead which could be neglected. An important point, though, is that it is for Gmail which is not necessarily a typical Web application.
The CPU overhead for SSL is small. A basic PC CPU core can do AES encryption at more than 100 MB/s. The cryptography which happens during session establishment (the "handshake") can be done several hundred times per second with the same kind of core, and it does not occur often, thanks to both HTTP keep-alive and the SSL "abbreviated handshake". It makes sense that properly implemented SSL won't clog the server CPU for most kinds of Web sites.
Network overhead for a single connection is also slight. When in full swing, a SSL/TLS connection breaks data into individual records, with a header and some bytes for cryptographic support: 5 bytes for the header, 16 bytes for the per-record IV if using TLS 1.1+ and AES/CBC, 32 bytes for a SHA-256 HMAC, and 1 to 16 bytes of padding for alignment. At most 69 bytes. But the record can embed up to 16384 bytes worth of application data, so we are talking about a 0.4% extra bandwidth. Again, that's negligible.
However, there is a site-wide network overhead due to caching. When the same piece of data is to be downloaded by many distinct clients, it is worthwhile to uses caches, located between the main server and the clients: when one client connection, going through a cache, is for a file that the cache has, the cache can answer without having to disturb the main server. The client and server needs not be aware of that mechanism; that's called a transparent proxy. Many ISP run that kind of system.
Caches won't help much with Gmail, because each user has his own mailbox and there are few files which will be shared between distinct users. However, the situation with Ebay may be different, with all the item photos that they send, by definition, to a lot of clients. It is highly plausible that caching benefits much more Ebay's infrastructures than Gmail's. However, and that's the tricky point, SSL prevents most of this caching. By going site-wide SSL, Ebay could possibly incur a non-negligible impact, enough to give them reason to pause and think. I don't say that Ebay's not doing site-wide SSL is proven to be a question of cache-related performance, but at least it is a possible reason.
Of course, not doing SSL means that passive eavesdropping and active attacks are a possibility. From Ebay's point of view, there are trade-offs everywhere between security and usability. Ultimately, as @Lucas says, it is all about money.