The cheapest and easiest route is to go from a 15T front to a 14T front sprocket. Front sprockets are relatively cheap, $20-$40 depending on brand. You will lose about 10 mph top end. You don't need to get a new chain. Going down one tooth in front is about the same as going up 3 teeth in the back. You can usually get by with going up 2 teeth in back using the stock lenght chain. Probably the maximum you can go without getting a new chain is down 1 in front and up two in back.