Hi Sarah
I can only really comment about Easter Island because I haven't done the main part of your tour. The Island has dual currency (Chilean pesos and US dollars) but the locals prefer pesos. It is a quite expensive island, mainly because practically everything has to be flown in so even a snack lunch of a sandwich and a drink will probably cost you close to a tenner.
In terms of souvenirs, again it can be pricey but a lot of items are made by local craftsmen and, of course, are pretty unique. There is a big craft market on the main street but I would also recommend the stalls in the café building at Ranu Raraku (the moai query) where you will find carvings and beautiful silver jewellery. If you want more traditional souvenirs like t-shirts, scarfs etc, 'Hotu Matua's Favourite Shoppe' (I kid you not...!) on the main street has the best quality but, like everyone else on the Island, they have slightly wacky opening hours! (Hotu Matua, btw, was the Island's legendary founding father and 'Ariki' i.e. the King.)
As a tip, although some people don't like to carry much cash, I would recommend it at the markets because the stall holders will usually give you a 'deal' if you pay in cash and save them from paying commission if you use their card machines. (Most of the locals only speak Spanish or native Rapanui but your guide may be willing to help you) If you buy anything fragile, make sure it will fit in your hand luggage because the Latam ground staff have a habit of insisting that wooden carvings (for example ceremonial paddles etc.) are taken off you if they spot them and put in the aircraft hold on the way back to Santiago.
Happy to help if you have any other questions
Have a super trip
Mandy