You're smart to stick in Green for budget purposes. I find Green has a lot of powerful cards that don't cost a lot of money to get.
If you're looking for a powerful and inexpensive commander, I always suggest [[Kamahl, Fist of Krosa]].
Kamahl himself isn't a super expensive legendary (around $3-4) and his effect is game-changing. His second ability, (2GGG: Overrun), is almost always a game winner when you have ten mana and even as few as three attackers. His first ability (G: Land becomes a 1/1) can be used to give you more attackers, OR, if your opponent plays a boardwipe, wipe their board of lands. If you're playing against big-budget decks, they're bound to have a bunch of boardwipes, and if your board state is threatening (it will frequently be), always keep some mana back so they know you can screw their mana base up.
Mono coloured decks are usually quite effective and efficiently-priced (albeit more narrow than multi-coloured decks). As you all stated, the newest pre-con EDH decks are pretty powerful out of the box. Not only do you have a smaller card pool to play from, your land count basically consists of 36 basic lands etc. and MAYBE [[Nykthos]]. You can do well even with only basic lands.
Deck structure-wise, I'd suggest playing a blend of big creatures, token generators, and mana sources. [[Terastodon]] is a very big creature which can ruin someone's day OR give you a few 3/3s. [[Wolfbriar Elemental]] and [[Protean Hydra]] can be great mana sinks when you've got a lot of mana and need some immediate power. [[Vernal Bloom]] can double the amount of mana you tap for. While it can help your opponents, it won't help them as much as this'll help you (unless they're playing mono-Green as well :P). [[Karametra's Acolyte]] is a very powerful ramp spell that usually puts Priest of Titania's mana-tapping ability to shame… I also suggest playing Priest plus a bunch of mana-dorks (like Llanowar Elves).
Overall, if you want to make a good budget deck, start looking for single-coloured legendary creatures, and then check out what's the best cards in that colour. You can always make a threatening deck, no matter your budget.