Unless you're wedded to the idea of bus travel across Peru, I'd be tempted to suggest a flight straight to La Paz. If you're spending any time in Bolivia, you'll spend a lot of time on buses anyway and flights there are more expensive than Peru and few and far between. From La Paz you could do a circuit on the bus to Santa Cruz via Uyuni, Potosi and Sucre, stopping off at each and getting to know some beautiful places, instead of spending several days on the bus crossing Peru.
LAN has flights from Lima for $299, while Peruvian Airlines do return tickets to La Paz for between $200-300. Depending on when you travel, the price will jump about a bit, so try combinations of days if you're after the best deal.