Rough costing and such.
Case: $39
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146076Case be sure it has openings for at least four 120mm fans. Fans are about $5 each.
Rear fan exhausts the case.
Top fan rear exhausts the case.
Front two intake.
Best case for the money. Aluminum so it is light, ideal quantity and placement of 120mm fan ports, comes in white and black, button quality is the best I have encountered, perfectly designed to attach Swiffers as intake filters, acceptable but not great cable management, looks great, and again cheap. Oh and did I mention bigfoots like them.
Power Supply: $69
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139050Best brand out there.
It is often up for a coupon code knocking the price down to $50.
500 Watt and modular to help with cable management
Mobo: $99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157370A good solid motherboard. Note does not have onboard WiFi.
It is often up for a coupon code knocking the price down to $90.
Memory: $65
http://www.newegg.com/Product/Product.aspx?Item=N82E16820313344If you watch you can get the same spec for up to $20 cheaper.
Be sure to get 8GB in two sticks.
Four is too little these days and two sticks allows for upgrades if necessary (it won't be).
CPU: Intel Core i3 to Core i5, 1150 Haswell, $124 to $189
This can always be upgraded, but a core i5 is a great long term answer.
Video: $150 -$200
Example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150702Example 2:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487028Sounds like a lot, however you have to remember that this card is a GPU, mobo, memory set, and cooler all in one. Also if gaming it does significantly more than the CPU.
The rest is up to the user...
Case fans
Optical Drive
Storage - HDD or SSD (I still like a 1TB HDD)
Monitor (it is wise to be sure it is a 1920 x 1080) any size is fine
Keyboard is a personal choice
Mouse is a personal choice (be sure it has forward and back buttons)
This should get you a solid build that has several years of useful life. As it ages you can always update the graphics card.
If you connect to the router via WiFi dongles usually suck.
Consider this...
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131979Amazing connectivity. Best I have found to date.