to save money when I visited, I couchsurfed and that worked out well. People were very nice and offered to drive me to the prison. The ESTA visa costs $14 dollars. Aas for prepaid cards, beware that the card issuer will charge you loads when you withdraw. Cash is a good option. Also, prepaid cards are not accepted online, at self checkout machines, for values under or over a certain amount (specified by the retailer) and usually require you to sign a receipt to they make sure it's you, so it can be a bit annoying. When I travelled in April the dollar was almost 2 for the pound, so I used way less than I expected. Plus I found american things to be a lot cheaper than in the UK. Renting a car went from 17$ a day, I thought that was insane. Also, I booked flights searching with google and got pretty good prices, a return via iceland with a cheap icelandic company was really worth it. I'd also check for united flights, because regardless of where you fly to, changing your flight to any other US destination costs 80 pounds, so it may be worth buying a ticket somewhere else and changing it. It's what I did.