I'll be basing this off Tech Report (since they have a scatter plot of perf/price)
http://techreport.com/articles.x/21813/19
FX-8150 at $245 is 355% ($0.69 per percentage point)
FX-8120 at $205 is 330% ($0.62 per percentage point)
Core i7 2600K at $317 is 425% ($0.74 per percentage point)
Core i5 2500K at $216 is 360% ($0.60 per percentage point)
Looking at them in terms of price per performance, they're not that bad.
However, the 8150 can by no means match the 2600K In pure overall performance (425% v. 355% = 70% difference), and is actually behind by 5% to the 2500K while being a bit more expensive ($0.69 v. $0.6).
The 8120 is even worse, just add $11 and you get 30% more performance (at $0.37 per percentage point) with the 2500K.
And there is still the power consumption to talk about. Core i7 2600K and i5 2500K both idles at 64W. Peak power consumption is 144W and 132W respectively. The FX-8150 has an idle power consumption at 76W and peaks at 209W. There is also a "task energy" graph for them; 8.5W and 9.9W respectively for the two Intel CPUs while it's 14.4W for the FX-8150. Comparing the 2600K with the 8150, $317:$245 means you save $72, but you end up using more power (12W more at idle, 65W more peak, 5.9W more task energy).
If the 8150 and 8120 is priced at $200 and $165 respectively, then it would be $0.56 per percentage point for the 8150 and $0.50 per percentage point for the 8120. This would offset, at least, the raw performance advantages of the 2600K and 2500K respectively.
Take note that those performance percentages are as "percentage points." Meaning, "behind by 5%" doesn't mean that it (8150) is 95% the performance of the 2500K (355/360 = 98.61%). And "30% more performance" is "30 percentage points more in terms of performance."