This may be helpful. I've been checking flights from UK to Santa Cruz for April next year and apart from American Airlines (not my favorite airline) there was nothing under £900 - including trying to go with Air Europa via Madrid. Suddenly 2 days ago Air Europa fares dropped £200 to £701 ret from Gatwick to Santa Cruz via Madrid. A bargain!

i recently had an air europe flights SC - Madrid - SC for about that price booked and they went and changed the dates of my flights by 2 whole days without even telling me causing me a lot of problems (including me having to change my connecting easyjet flight dates) but also causing us lots of paper for visa reasons.
So im not saying dont use them , but best to be aware that they do change their dates around a bit sometimes without telling you - keep your eye on them if you do book. I ended up canceling my flight and booking with BOA for about the same price but actually leaving from and returning to La Paz which was better for me anymore.
Air Europe and BOA are definitely the two cheapest options from Europe though.

If you arrive in Santa Cruz by Air Europa with an onward connection make sure you have plenty of time to catch your next flight.
I came in from Madrid yesterday. The flight was an hour late, but still only 06.20, so I should have been able to catch one of the early morning flights to La Paz by Amaszonas or BoA. However, there was epic chaos in the baggage claim area at Santa Cruz. Only one carousel and a very small area, wedged in between the immigration desks and the customs lane. Totally inadequate for the baggage from an Airbus 330, particularly as no self-respecting Bolivian returns from a trip abroad with less than two large suitcases and probably a plastic-wrapped stuff bag or two as well. not to mention folded push chairs. Only one customs officer on duty - working womanfully to clear the increasingly restive queue (well, crowd of people).
I eventually emerged from customs at 07.50, and there was still a long queue behind me, with the bags from the American Airlines flight from Miami already coming through.
So be warned.

New Air Europa advertisement in the Bolivian press suggests some arrangement for connecting flights from Santa Cruz to other cities in Bolivia has been made with Amaszonas.
No details available on either Air Europa or Amaszonas web pages. However, this might improve the connexion time in Santa Cruz.