I second the idea that you consider more than just the frame material.
Carbon fiber does offer the greatest strength for the lowest weight, ultimately ... but a cheap CF frame might be dead (unresponsive) and overweight, thus offering none of the benefits of the material. A really good Al frame with a CF fork can be built lighter than a cheap CF frame and offer all the performance and comfort one could desire.
If you are looking to shave ounces, whether it be for competition or just personal pleasure, then you will need to go with CF, likely. You will probably be able to find a lighter CF frame at a lower price point, because that is the way the industry (except Cannondale) has gone. Cannondale still offers its CAAD bikes which are as good as CF in every respect, pretty much ... unless you are going for Absolute weight-savings. But still .... Are you considering Solely the weight of the frame?
it could be that a less expensive Al frame could be paired with lighter wheels, for instance, so the overall weight might be lower than a CF frame with heavier components.
I get the impression that you are looking for a bike to buy and ride---not a bike to buy and then modify at considerable expense (you would be hard-pressed to find light wheels for under $300, and there you might only be saving a few hundred grams over stock wheels.)
Further, unless you are riding pro, in which case your ride is sponsored ... 100 grams is only going to matter on paper. Not saying weight doesn't matter---man, has that idea been thrashed to meaningless here before---but that a really minute difference will only make a really minute difference. In most cases, going to lighter tires and tubes can gain you a hundred or a couple hundred grams .... and replacing the stock seat post and saddle will often save much more than that.
So ... if you are set on owning Carbon Fiber, for whatever reason,. Bikes Direct, and leftover models at local shops ... two best routes, IMO.
If however, your goal is to buy a Lightweight bike ... then stop looking at frame material. Look at overall weights (generally much harder to find, sadly) and go with the bike with the best components at your price point.
Another option is to look at bikes a few hundred dollars Below your maximum price, and budget in a set of Vuelta (for instance) wheels, lightweight tires, and a lightweight saddle.
Either way ... first you need to decide if you want CF or low weight--the two are not synonymous. Then you need to decide the absolute max you are willing to spend.
One final note---frames matter. if you get a cheap CF frame, because it is CF, then the frame might not be worth hanging a bunch of really good parts on. A very good frame, whatever the material, can be kept for a Long time, which means it will be a good investment---as will all the upgrades you will likely buy as time goes on. (Giant, for instance used to have a reputation for offering frames better than the components, which made later upgrades a good deal. No idea if that is still the case.)
You need to decide of you are going to buy a bike and basically do nothing but ride it, wash it, and lube it for the next five or ten years .... or if you might start replacing some of the components over time.
If Bikes Direct, for instance, offered a cheap bike with really good components (say, a heavy-ish Al frame with 105 or Ultegra---generally found when you buy the top of the line of a bottom-of-the-line bike) and also offered, at the same price point, a better bike with lesser components ... you can buy a 5800 105 group for about $350--$400. Ride the good frame and wheels with Sora or Tiagra for a couple seasons, buy and install the 105 group, and you will be happy from initial purchase until you retire the bike because You are too old to ride it.