Read these reviews. It's evident on the trip that many don't, but exposing yourself to the advice provided from people who just like you, wanted to trek to Basecamp, can only enhance your trip- even if you choose to ignore the advice, it gives insight into what to expect.
Be generous; yes, food costs more higher up- but look at the loads the Sherpas and porters are carrying; look at the tracks you are walking on, and think about how tired you feel on this holiday. Don't be mean spirited and deny the vendors their right to charge a little extra. If you want to get the Exodus yak to drag your stash of home bought goodies up the mountains, then I hope you have fun in your teahouse room being sad that your mars bar is crumbling and broken.
Others have commented on this in their reviews too: any unwanted trekking gear/ clothing/ footwear is very, very happily and graciously received at the end of the trip. Exodus have a system in place for this, and their porters make use of these items immediately.