Since you're real new to backpacking, it strikes me that you might want to start out with a somewhat more modest shake-down trip, as soon as you can do it, perhaps a week or two. With as much borrowed/rented gear as you can so you don't commit to buying stuff and then regretting.

"More concerned with overall cost than with weight": I suggest that you rethink that, and be concerned about both of them. If you're out for months, spending a little discretionary money --- or time --- to work towards a lower base weight makes a big difference in a pack that you'll be carrying much of the day for months. Look into MYOG forums here and on BPL (backpackinglight.com) as well as for gear sales to see where you can make or buy lighter versions of things for less money.

Since what you're doing is along the line of thru-hiking, I suggest that you look at some thru-hiker gear lists to get a sense for what they carry. The emphasis tends to be on "just the gear I need, little extra weight" --- except typically a couple of personal luxury items that vary with the individual. Just as a convenient (for me) example, here's my starting gear list for the CDT this year: http://www.nwlink.com/~brianle/Gadget_CDT_2011_Gear_List.pdf
This was for an early season start (lots of snow) in Glacier National Park in June, and came out to around a 20 pound base weight (weight of pack minus consumables --- food, water, stove fuel if a stove is carried). By the time I finished in New Mexico, my somewhat wild guess is that I had pared the list down to perhaps a 16 pound base weight, maybe 17 tops. But as water sources are limited as Lori said, and IF you start out armed with good info on where water sources are available (easier on an ~established trail as I was on), then you hopefully won't be struggling under heavy water loads a lot. But it purely sucks to be out there low on water (!). I really have no idea how you would get such information, however. Would you be planning to jump on to private property all along the way to raid windmills?

Anyway, w.r.t. the gear list, one thing you want to look at your gear list for is "completeness". Look at my or other lists and see if you see needed items that you forgot to put on your list. It's your job to review your list for completeness, it's too tedious a task to ask internet strangers to do that for you.

As far as "how far off base are you" --- my intuition is that you're trying to bite off more than your experience base prepares you for. The saving grace is that since you plan to stick to roads (ugh, most thru-hikers opine that extended road walking sucks ...) if you get in trouble it will hopefully not be too hard to hitchhike to somewhere or beg help off of passing strangers. But maybe not, the CDT does a lot of dirt road walking where the roads are very lightly travelled if at all in New Mexico.

Another whole vector to consider are private property issues. If you walk highway, too often it's fenced on both sides of the road; where will you sleep? If bad luck and circumstances require a person to make a no-trace stealth camp on private property one night, that's one thing, but planning to do it every night would be quite another, I think. And FWIW, make property owners even more leery of us scruffy backpacking types. Please in any event don't light fires if so ...
If you walk more back roads, a whole lot of them will have barbed wire fence gates across with "no trespassing" signs. Leased BLM land gets a little ambiguous, often it's tough to know if you're legal on a county road or are indeed walking right onto someone's private land.

I suggest that you think about why you want to do this particular trip; consider equipping for and doing a chunk of, say, the PCT, where you can sort of learn as you go from other long distance hikers along the way. It's very helpful on your first very long journey to have at least some sort of minimal support system like that.

To be clear, I'm not meaning to be patronizing or discouraging, just bringing up issues to consider with my sort of hip-shot reactions. Best of luck whatever you end up doing.