I levelled mine in a few hours before I levelled mining on the same toon, would have been cheaper to do it the other way around.
It cost about 5k including buying all the mats for jeeves. But I think it would be less now given the servers economy that I did it on. Mining would probably decrease it considerably.
I don't pvp on that toon, but I see people use engineering when I pvp on other toons. Goggles are great since there is a resilience cogwheel, but you will replace with vicious helm.
Its incredibly easy to lvl IMO, but you really should use a miner unless you're like rollin in the 100kG club. The revamped azeroth is filthy with ores now (I'd almost say double what you could find before), the only lull you'll hit is BC and Wrath stuff. 475 on was a breeze because I think almost every recipe you use gives you 5lvls.
I'd go with JC or Ench personally so you have one prof with good CD's and one with passive stat boosts and profit potential (since eng practically has none.)
Not really sure about pvp personally, it's fun in non rated bg's but I don't think you can use most of the stuff in any rated gameplay.
yeah when cata hit powerlvld from 450 to 525 10kg
rocket boots are really nice for pvp and you can put two 208 haste / crit / mastery / resi gems into the helmet
i would go with jc or enchanting as second prof
Engineering is a bit of a pain to level when below 250 (the Thorium widget/tube recipes are available at 250+). You'll have to get various pieces of cloth, leather and gems and you'll be stuck with only yellow recipes at times. Although the abundance of low level ore makes up for it. I managed to farm 160+ Thorium ore in a single Winterspring lap along with a fair amount of gems and 200+ Dense stone (which I mostly vendored).
The Fel Iron and Cobalt part of engineering can take quite some farming. The Adamantite and Saronite (Saronite = Titanium if you have a 450 Alchemist) parts are very easy to farm for yourself if you want to.
I wouldn't know how much it would cost, since I tend to farm everything myself (I keep a stockpile of mats at all times).
Good professions to go besides Engineering are Alchemy, Inscription, Jewelcrafting, Blacksmithing and Alchemy. All of those will give you a flat 80 points (81 for Jewelcrafting) to a primary stat of your chosing. If you're looking for a profession to supply you with mats (mining), get that profession on an alt.
As for PvP, there is rated and non-rated. In rated, the On Use effect of the Synapse Springs means you can use it to along with other major cooldowns/procs for more effect than a flat 80 stat points that other crafting professions would give. In non-rated you can use other tinkers like rocket-boosts, parachutes and grenades to go along with the Synapse Springs, making Engineering even better.
I just bought everything I need on the AH for 3k and levelled it in about an hour on an alt - it's 100% subjective to your server economy though. Most expensive for my server was cobalt and saronite (Even though it's so easy to farm). I know on a lot of servers mithril and adamantite are pricey.
It was extremely expensive to level on my server, even with mining. Totally worth it though, specially for pvp. However you have to get creative in order to profit off it. Make guns and choppers