For something as "heavy" as 2.5 with a 30° limit, you should have LOTS of options. For starters, in the $100 range, you can find the Kelty Cosmic 20 or the REI downtime. Neither are 800-900 fill but are at or below 2.5 lbs and good well below 30°. I have the (old) Kelty Lightyear 25°. I have pushed this bag will below 20° and, well, survived, but 30° is a no brainer. Bought it on sale 4 years ago for $70 and it weighs in at exactly 2.5 lbs.
REI now has a weight based search component. If you look at prices <$200, there are plenty to choose from, but many are in the 2.5 to 3 lb range.
However, for just a little more, you can pick up a 20° Burrow from Hammock Gear for $229.00. It weights in at a stunning 21 oz and is a great product. I own this bag (long with 1 oz of overstuff, cost +$30.00). I love this bag! I can vouch that the rating on these bags is very accurate, not some manufacture-manipulated sales gimmick. These are serious quality products. hammockgear.com