1. Cheap blazer. Actually, it's an "inexpensive" blazer. Go to http://andersonlittle.com/ . Hopefully, you already know your jacket size. If not, swing by some dry cleaners that does alterations, and have the alterations guy (or woman) measure you.
Then run it past Anderson Little, and ask them if, based on your height and weight, the number makes sense. (Hey, it never hurts to confirm things.) Then buy yourself an A-L blazer. $179, and free shipping.
It's not a great blazer, but it's a perfectly acceptable blazer. It's also not particularly trim-fitting, but frankly that's a plus in my book.
Once it arrives, take it to the dry cleaners where you had the alterations guy (or woman) measure you, and reward that person by paying a few bucks to get the blazer's sleeves shortened to the right length. That's may well be all you'll need to have done to it to get it to fit tolerably well. It's an inexpensive alteration to get done.
2. Khakis. You could go to Sierra Trading Post, and try to figure out which pair of Bills Khakis fits you the way you want, and get them for 30% off the already reduced price by using a coupon obtained as a result of offering Sierra Trading your undying love on Facebook or something. Or you can go to www.landsend.com, read the descriptions of the various khakis offered, and buy a mess of them in all different sizes and styles, secure in the knowledge that you can keep the one pair that works best for you, and return all the rest to any Sears store for a refund, no muss, no fuss. Oh, and Lands' End will, in most cases, even hem/cuff the pants to your preferred length, at no extra charge. (I prefer them cuffed.) And there are always discount coupons for Lands' End. $10 off $50, free shipping, 30% off, etc.
I don't know how much this will cost you, but let's say $30-60, depending on exactly what you buy, where you buy it, and which specific coupon you use.
3. White shirt. The aforementioned Lands' End. I like the combination of a button-down collar with a blazer. But either way, Lands' End. It'll probably set you back around $35. More or less. It'll be a highly acceptable white shirt. Not the best, but inexpensive and entirely satisfactory for its intended purpose.
4. A tie. This one's easy. Go to www.thetiebar.com Narrow it down to a few ties that you like, and come back and ask for people's opinions, if you want. They have a zillion ties to choose from, so you may want to decide ahead of time what sort of tie you prefer. Silk knit, or "regular" silk. Stripes or geometric pattern. With a navy blazer and white shirt, there are LOTS of perfectly reasonable ways you can go here. Both of those clothing items are dead easy to match to a tie. Anyway, not the greatest ties in the world (not even close), but they are perfectly fine ties, and The Tie Bar offers a huge selection, and the price is a mere $15 per tie.
Assuming you already have an appropriate pair of shoes, you're all set. Under budget and everything.