Budget builds means your not looking at $300 I7's and 970 GTX's, your looking for the best bang for you buck. The Best time to consider a budget build is from Black Friday until the 1st of the year, that is the only time where you are going to be saving $100 to up to $300 on new build. This time of year you get much less for you money as buying new parts gets harder to find deep sales.
Here are some tips.
Consider Used parts and be patient.
1. Cases - Goodwill, Craigslist, Electronics Recycler are great places to look I have found excellent gaming cases for next to nothing.
2. GPU's, CPU's, RAM, Optical Drives,HDD, Monitors- Ebay, Craiglist and countless Classified Forums are the place to look Gamers are always updating so the used stuff is out and mostly less than a year old with deep discounts.
3. Power Supply and SSD- Avoid used always find a good quality unit on these two. https://pcpartpicker.com/ and https://pchound.com/wgWTrO/ will gather prices from online sources for you to compare.
4. Windows- Ebay,Goodwill and,Yard Sales, look for old PCs using Win 7 you can find them dirt cheap. Electronics Recyclers are also a great source this is where I get my keys. Remember anyone with a Valid Win 7 or Win 8 gets a free Win 10 upgrade so why buy Win 8?
Here is a example of my finds this week, my budget was $450 and the Client had nothing to start with.
I5-2500K, EVGA Z87 Classified, 8gb 1600mhz Gskill Ripjaw all for $140 off Tech Forum
A broken Compaq business Slimline with Win 7 Pro sticker.....Free from a Bank
24 Inch Asus 1080p monitor 3 years old $40 - Yard Sale
1 TB WD Black $25 - Tech Forum
Corsair Carbide 300R Mid Tower Case- $25 Craigslist
Hyper 212+ $5 Criagslist
EVGA SuperClocked GeForce GTX 760 2GB $75 Tech Forum
Samsung SSD 256 SM841 256gb $35.00 Tech Forum Low Usage with Receipt and unregistered
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$345 without a Power supply.