You will go broke if you pay to get into each museum, church, and archaeological site around Cusco . Even if you are going to visit only a few places, buy a boleto turístico for US$45 (US$24 for students under age 26 with ISIC card). The ticket covers 16 sites in and around Cusco, including must-sees like Sacsayhuamán , Pisac , and Ollantaytambo , and you cannot actually get into a lot of the sites if you do not have the ticket.
The tickets can be bought at the entrances to most major sites or at COSITUC (Av. El Sol 103, #102, www.boletoturisticocusco.com , 9 a.m.–5 p.m. Mon.–Fri.).
Unfortunately, the pass only lasts 10 days. There is a process for extending expired passes, but—take it from us—it’s not worth wasting a day navigating the halls of the Instituto Nacional de Cultura, which is a harrowing bureaucracy even by Peruvian standards.
The ticket covers these 16 sites:
The Catedral  (US$8.50), Iglesia de la Compañía  (US$3.50), Iglesia San Blas  (US$5), and the Museo de Arte Religioso  (US$5) are not covered either, but if you plan to go to all four you can buy the boleto religioso at the entrance of any of these sites for US$17 (US$8.50 for students under age 26 with ISIC card).