Tough criteria...

I'd think of having two sleeping bags, one lighter weight for summer, one heavier weight for spring and fall, and both for winter. Of course, you'll need a good sleeping pad too. Actually, a lightweight quilt and a sleeping bag might be better.

I'd also go with a large (8x10 ft) lightweight tarp and ground cloth instead of a bivy sack for luxurious space as well as flexibility. Learn how to pitch a tarp and you can use it under any conditions, winter or summer. It avoids most of the condensation problems and claustrophobia of bivy sacks.

In the winter, I'd also think about using a vapor barrier liner to eliminate condensation from destroying the insulating value of your sleeping bags.

So, my list includes:

1 8x10 silnylon tarp (or lighter but more expensive material).
1 lightweight quilt
1 good three season sleeping bag (most expensive item)
1 vapor barrier liner (inexpensive and lightweight)
1 ground cloth (inexpensive and lightweight)
1 or 2 sleeping pads

If you shop around some of the ultra-lightweight backpacking sites, you'll find stuff that meets your size and price criteria. A good 3 season sleeping bag, however, will be the most expensive item and the one you'll have to make compromises on.