I decided to do one of the more interesting ones. Something similar to the ST15 at $1300.
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Core i5-4690 3.5GHz Quad-Core Processor | $213.78 @ OutletPC |
| Motherboard | Gigabyte GA-B85M-DS3H Micro ATX LGA1150 Motherboard | $59.98 @ OutletPC |
| Memory | Team Elite 8GB (1 x 8GB) DDR3-1600 Memory | $53.99 @ Newegg |
| Storage | Kingston SSDNow V300 Series 120GB 2.5" Solid State Drive | $49.98 @ OutletPC |
| Video Card | Gigabyte GeForce GTX 970 4GB WINDFORCE 3X Video Card | $323.98 @ Newegg |
| Case | Cooler Master N400 ATX Mid Tower Case | $39.99 @ Micro Center |
| Power Supply | Corsair Builder 500W 80+ Bronze Certified ATX Power Supply | $29.99 @ NCIX US |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total (before mail-in rebates) | $866.69 | |
| Mail-in rebates | -$95.00 | |
| Total | $771.69 | |
| Generated by PCPartPicker 2015-03-06 21:54 EST-0500 |
That's about $450 markup. The product page says a 120GB SSD so that's all I included, but I'd expect them to throw at least a 1TB drive in there too. Add another $50 or so to that price and we still have a $400 difference.
Now, I used basically the cheapest parts I could find. If I were personally building this PC, I'd go with a 4690K, a Z97 mobo, a module power supply, and a nicer case.
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Core i5-4690K 3.5GHz Quad-Core Processor | $222.75 @ OutletPC |
| CPU Cooler | Noctua NH-U12P SE2 54.4 CFM CPU Cooler | $57.88 @ OutletPC |
| Motherboard | ASRock Fatal1ty Z97X Killer ATX LGA1150 Motherboard | $114.99 @ Micro Center |
| Memory | Team Xtreem Dark Series 8GB (2 x 4GB) DDR3-1600 Memory | $54.99 @ Newegg |
| Storage | Crucial MX100 256GB 2.5" Solid State Drive | $104.98 @ OutletPC |
| Storage | Toshiba Product Series:DT01ACA 2TB 3.5" 7200RPM Internal Hard Drive | $71.99 @ NCIX US |
| Video Card | XFX Radeon R9 290X 4GB Double Dissipation Video Card | $302.98 @ Newegg |
| Case | NZXT Phantom 240 ATX Mid Tower Case | $68.99 @ SuperBiiz |
| Power Supply | EVGA SuperNOVA NEX 750W 80+ Gold Certified Fully-Modular ATX Power Supply | $74.99 @ NCIX US |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total (before mail-in rebates) | $1179.54 | |
| Mail-in rebates | -$105.00 | |
| Total | $1074.54 | |
| Generated by PCPartPicker 2015-03-06 22:12 EST-0500 |
This is closer in price, but it's still $100 cheaper, includes a 1TB drive in addition to an SSD twice as big, and thanks to the unlocked CPU, would outperform the Steam Machine, especially once you did a little overclocking. The bigger case would give you more room for airflow letting you push harder without having to worry about thermals.