Re: the pack-
What do you consider low cost? In that volume range, I absolutely love my Conduit (I'm turning into an ULA pimp here), priced at $125 before options. Also, the Jam2 is right at $100 and is 3000 c.i. Both are primarily Dyneema so they'll be plenty durable and both would cut about 20-25 ounces from your gear weight.
Re: gear list-
As pointed out, the Atom might be too much for low desert hiking spring-fall and with only a half-zip, it could be very limiting. Of course, its also you're most expensive piece and you're on a budget. If you're not going to see temps under 40 degrees, I'd sell the Atom and get Jacks R Better's Shenandoah quilt. You'd need to add a fleece watch cap but you'd have more warm-weather versatility at about 2 ounces lighter weight. For temps under 40, you can always add a high loft vest/pullover and poly longjohns.
Are you pitching the tarp using natural, site-located supports or are you using poles? You might want the flexibility that trekking or carbon fiber poles would grant you in selecting camp locations.
You'll probably want some sort of brimmed hat and possibly l/s shirt for sun exposure. Possibly you're not counting these items here as they'll be worn. For hot nights, I also like silk underwear instead of skin against nylon bag fabric, but that could be considered luxury, I guess.
If you don't plan on hiking at night (and possibly even if you do) you can probably ditch the Ion. I always carry a headlamp, but I don't use a pinch light.
Camera? I also don't see route finding items or first aid & toiletries listed, but I assume you know what you need there. Little things like stuff sacks, first aid, etc do add up.