Hi there,
You can rent a lancha or small passenger boat in Puno with the locals on th same day. Rates are by the hour or day. This is the most economical way to see various sites around the lake. The great thing about this, it's that you can talk to the captain and customize the excursion to fit your individual needs.
To give you an idea of prices, you can secure transportation from
Cusco to Puno for as little as $5 on a bus, obtain a lancha to
the islands for $12, and book lodging in a Lake Titicac hotel for $20 or a hostel for less than $10 per night. Standard rates for meals in the area run $2.50 for an American breakfast, $3.00 for a fixed menu Peruvian lunch, and $8 for a full course Continental.