if you ship a lot of items you can negotiate the price per pound with your courier. other than this, what winde said. some mudanza guys have flat rates for boxes but it only refers to miscellaneous stuff like food, items of clothing, some cosmetics and such. they will have a different rate if your box is to contain a single electronic item, for example.
Shipping by boat with EPS or Encargo pac, the price is roughly 33% of the item price. Takes longer but much cheaper. I just brought in a manual lawnmower weighing 26 pounds and shipping was 1300 pesos through Encargo pac.