OK, guys from personal experience. I'm evaluating a new lighter weight sleeping bag/or system that won't break the bank.

Bite the bullet and buy which of the following and why?

Marine 3 season system (bag and bivy)?

Snugpack Sleeplite (what bivy?)

Stay with GI sleep system "patrol bag," and bivy?

Are there other economical (good enough) options?

Light plus warm?

Good down bag, bivy cover, drybag for transport.

Won't be cheap though.

I have a minimalist bivy by REI I got for dirt cheap on sale. It's lasted 10 years and lots of trips. I use it with winter and summer bags.  My summer bag is a synthetic by marmot, my winter bag is a down bag from Big Agnes. Both are awesome.

Brain fart

US Army sleep system for economy and warmth.

Just use the "light" bag out of the kit.

North Face "Cat's Meow".  Synthetic, rated at 20 degrees, under 3 lbs, weight savings adn stuff size kicks the pants off the MSS, In the $150-under 200 range depending on where you buy.
Shop at Backcountry.com, Steep and Cheap, Sierra Trading Post, campmor, REI, or other online dealers.  You can get last years models for a good deal.
If you're looking for performance and weight savings, mil-surp should be the last option.  It's built heavy to last over varied conditions, stressful situations, multiple users, in a one-size fits most world.
The commercial market is where it's at...but it'll cost ya.

you cannot beat the issue sleep system.  You can find them for about 100 bucks new and unused.

USMC Improved 3 season system - I just learned about this recently and have read nothing but great things about it. It's light, pretty packable, and warm (not 4 season), but they're pricey, about $350.

Snugpack SleeperLite + Bivy - I had a sleeper Lite, nice bag, solid rating at 32F though I sleep cold so it was more like a 40F bag for me. It's only about $60 and you could pair it with the USMC bivy for only another $50+ on ebay

USGI MSS it the cheapest option, warmer, but also the heaviest and bulkiest if using the black bag. If only using the green bag it's probably not as warm. The bivy is heavy at 2lb+ and not as nice as the USMC IMO. But it's tough to beat $100 for a bivy, 2 bags, and compression sack.

It just depends on what you want.

You also have some nice civilian options if you want light & warm and don't mind spending in the range of $200.

It's great for what it costs, but it is damn heavy.

Usgi sleep system bivi and a snugpak here.