A maximum group size of 16 men, Transportation to the beginning of the trek, entrance fees for the Inca Trail and Machu Picchu, Guide, Tents (2 persons per tent), Dining Tent (communal tent where the group will eat meals and for the porters to sleep in at night), Kitchen Tent (to prepare the meals, store the food), Meals (and a cook to prepare them), Porters (to carry just the tents, food, and cooking gear), Emergency oxygen bottle and basic first aid kit. Easy arithmetic can show that these companies have to depart with the maximum of 16 individuals each trek (consisting Inca trail of customers from various budget agencies) and that they cannot supply a quality service or pay their porters half as much as they should be paying. After paying several hundreds of dollars on your domestic and international flights to see with Peru why compromise on the quality of your trek and the treatment of the porters for the sake of saving USD$50! Sometimes these companies can be of a lower standard than you've signed up to.
Purchasing a trek directly with a local tour operator like we did based in Cusco such as Kuoda Travel, offers much better value for money. There are over 200 tour businesses based in Cusco alone and the wide variety of similar sounding names can be confusing. I have looked at the Andean Travel Web Guide and I have attempted to check reviews here (but the trekking group reviews are all over).
To reserving a trek in your own country the clear difference is a daunting task especially if you do not speak Spanish and that merely phoning the company to make a reservation can not be cheap. It may also be a place filled with misleading advice, phantom companies and businessmen simply outside to make a quick buck although the internet is an excellent source of information. As discussed above, attempt to establish the maximum group size, whether the trek departure is ensured (you don't want it to be cancelled at the last minute if they don't meet the minimum number needed), if the guide speaks English, what is the food like, and what train service (if at all) is included.