There have been threads on this subject, but it can use updating anyway.
The first item that is the lightest and least expensive: knowledge, training and research. For example, if you know the principles of layering, you can put together a good clothing kit at very little expense. It might weigh a bit more than the highest performance UL clothing, but I'll bet I can walk into a thrift store and come out with a perfectly good layered clothing system that is maybe a pound more than the high priced spread– and 10% of the cost.
My favorite ways to save:
Used over new, deep discount sales over retail. Yard sales, thrift stores, Gear Swap, eBay, REI Garage Sale and Craigs List rule. Anything and everything can be found used. Caveat emptor!
Old ski poles for fixed trekking poles. I wanted to test the concept and my first poles were a pair of $5 yard sale downhill poles. They were light and very strong. Used cross-country poles are even better.
Recycled water bottles vs. Platypus or Nalgene alternatives
Pop can alcohol stoves and IMUSA or Foster can pots
Cheap blue poly tarps are just a few ounces more than a PU coated nylon equivalent
Ponchos or DriDucks for rain gear
Trail runners for foot gear
Running wind pants and jackets
Mora knives; Victorinox paring knives
Blue foam sleeping pads
Bic lighters
Recycled plastic spoons, bowls and cups
Buy irregular socks on sale at 50% or better (REI and Sierra Trading Post)
Fleece is a little heavier, but can be very inexpensive and perfectly good insulation, even better in wet conditions.
Military Polartec Power Dry fleece– just like Patagonia R1
Polyfill sleeping bags and garments tend to cost less
Windfall finds like the $15 JCP down jackets
Some retailers take in used items for trade. Second Ascent in Seattle is my local example.
Have a yard sale of your own to generate funds for new purchases.
MYOG can save, but there are some roll-out costs.
Good cheap insulation is a tough one— jackets and sleeping bags. Used is the first level, with heavy discount sales next.
Shelter is about the same as insulation. I have an older REI Half Dome 2-person that I use for car camping. I bought it for $20 at a yard sale. It is six pounds, which isn't bad split between two people. It is a very good shelter and someone on a budget could bear the extra weight if they have the rest of their kit in UL order.
IMHO, any gear list can survive one heavy item. Make that the most expensive item on your list, to be whittled down as opportunity knocks. It's rather like buying your UL pack last. I assume that would be one of the big three. Patience pays!