Few words about costs:
Along the popular routes which require no compulsory guide (Annapurna, Langtang-Helambu and Khumbu), daily food and lodgings cost $15-30/day depending on eating habits and area, Khumbu/EBC being the most costly. At those prices it is actually cheaper to trek than spend time in cities. As it is perfectly possible to trek alone that is the total daily cost.
Always in addition comes the fixed cost of different permits and passes/tickets, which usually cost around $50/trek, no matter how long it is.
If one wants to hire a porter-guide, who carries maximum of 30kg load and is able to communicate in english, the cost is around $20/day + 15% tip at the end. Transportation must be also provided. As one porter-guide can carry for two persons, total daily cost for a couple employing a porter-guide is about $35/day/person.
For a larger group it is practical to hire a guide who speaks better english, but carries no load. Daily cost is $25-30/day + 15% tip. This cost is divided by all in the group. Than one porter is needed for each trekker, at $15/day per porter or $7.5/day/trekker.
Those prices are for groups or individuals who want to arrange their own treks for complete independence and freedom what comes to routes and timetables.
There are also readymade packages, where one can buy a slot on a pre-arranged trek. Those cost from around $60-100/day, where food, lodgings, guide & porter service is included, also transportation to/from trailhead and all necessary permits. These are sold also by western travel agencies, who subcontract the actual services from Nepalese agencies, adding their 50 something % to the price. If one buys directly from the local agencies the prices are as above.
The last kind of trek is the camping trek, where there is a whole retinue of guides, cooks, kitchen boys, porters etc, and trekkers sleep in tents, eat in a restaurant tent, use a toilet tent, etc. Those usually cost $120-250/day and are totally unnecessary along the popular routes. Agencies and travel agencies love to sell these to unsuspecting customers, of course, even when lodge based accommodation would be more comfortable and much cheaper. It is a different matter along some more remote routes where there are not enough lodges or not at all, and local communities can not provide food for trekkers (Dolpo is a prime example of this kind of region).
I have once been a "member" on a 32 day camping trek, where we had 31 staff for 7 trekkers. Guide, 2 assistant guides, cook, 2 kitchen boys, 2 kitchen helpers (there is a status difference…), sirdar (porter leader), porter cook, porter cook kitchen boy, and finally 20 porters (one of which carried only eggs). Total cost of this, including chartered bus to trailhead and permits, was 47€/day per person, about $60. It was all arranged directly with a local agency cutting all middlemen from the equation (sometimes 2 levels of travel agencies adding 40% commission each). Thus even exploring the restricted areas need not be hugely expensive, when you know the ropes.