There are a number of sources for inexpensive lightweight gear and clothing; here are a couple of good references:
Try Mark Verber's encyclopedic website:
http://www.verber.com/mark/outdoors/gear/index.html
It lists lightweight, low budget options at the end of each gear and clothing category. He also has a section called "Backpacking for Cheap":
http://www.verber.com/mark/outdoors/gear/cheap.html
with links to a number of gear lists, suggesting many items which can be bought or made quite cheaply.
Another favorite of mine is "Sgt. Rock's" "Dirtbagging and Deal Shopping":
http://hikinghq.net/dirtbag.html
Watch for sales! The google shopping feature is a big help. REI should have a sale later in January and always has the 20% off regular price coupon (one item only) for members in March.
Basically, the two items you list where you could save quite a bit of weight for the least expenditure are your sleeping pad (the most important) and your tent. Here are some specific suggestions:
Pad: Since you have a thick self-inflating pad now, you probably don't want to revert to the old blue foam style, a Ridgerest or even a thinner Prolite (you might try the latter, though). I suggest an insulated air pad, 2.5 inches thick or more. One to consider is the Big Agnes insulated air core which is nearer 1 lb. (depending on size). Do a google search and use the shopping feature; most sizes are under $100 and there seem to be a lot of sales right now. The Exped UL7 is getting many highly favorable comments, but it's a bit more expensive. Consider getting the short size and using your pack under your knees and feet (try this out at home first). Do try any pad out on the floor (not on carpet) for a couple of nights and send it back if it doesn't work for you–we each have different sleeping styles! With an air pad, the trick is not to blow it up all the way, but have it a bit squishy–find the "sweet spot" between too firm on the one hand and having your hip and shoulder bones hit the ground on the other. You may find yourself ditching your Base Camp, as I did mine 6 years ago!
With two of you splitting the weight, 3 lbs. of tent per person isn't quite so bad, but if you can find a tent that weighs 4 lbs. or less that won't break the bank, that will definitely help with pack weight for both of you (i.e. you can carry more water!). There are lighter tents, which can be bought used; look at the websites for Tarptent and Six Moon Designs and watch for used models on Gear Swap. Here's a Tarptent Squall 2 (2-person) for $150 on Gear Swap, which weighs 34 oz:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=57557
The specs are here:
http://www.tarptent.com/squall2.htm
I own one (my original lightweight tent, now used when I take a grandchild out) and really like it. However, it's not a freestanding tent. Tarptent's Double Rainbow can be made freestanding, if a used one turns up. Note, though, that you should stake down a freestanding tent anyway, lest a wind gust send the tent over a cliff or into the river! At least tie it to some rocks.
With "mainstream" style tents, watch out for "creative weights"–what you want is not the "trail weight" (which omits minor items like stakes, the stuff sack and sometimes the poles) but the "packed weight." Watch for sales, especially this month. If you can't find a tent weighing 4 lbs. or less, though, saving less than 1 lb. per person over your current tent is probably not worth the money.
Sleeping bag–If you want a new bag, BPL gave a good review to the Kelty Cosmic Down 20* bag, which can be found for $100 or less. It will be warmer than you need for the GC (although on the North Rim in May it might be just right), but if you can't afford a whole series of sleeping bags, this will work as an all-purpose three-season model. Just ventilate it (open zipper) or use it as a quilt in warm weather or (if it's too hot) sleep on top of it. However, the Kelty bag will save you only half a pound, and your current bag should work fine, so IMHO this should not be a high priority purchase for you. Down is quite a bit more compressable than synthetic if space in the pack is a problem.
+2 to David's comments on trail runners! I was a long time ditching boots–finally did it three years ago and haven't worn the boots since!
