I never thought I would find a brush that could compare to my Sigma F80 flat top until I tried the Physicians Formula Kabuki Brush. This was truly love at first swipe. According to the packaging, it's the "Softest Brush Ever" and I totally agree. Its flat shape and ultra dense bristles is perfect for buffing your foundation for a beautiful airbrushed finish. I myself prefer to use cream or liquid foundation and this has worked very well in smoothing it out. The density and feel of this brush is pretty much the same as that of the sigma F80 so there is only one thing that makes this brush stand out more (especially for a bargain shopper like me) and yup, it's the PRICE. This brush retails for about $10-$11 but if you have a ROSS in your area then you might even be able to get it for $3.49. Now if you can't get to a ROSS store or have no luck finding one, here is a deal idea that you can do at Ulta.
Physicians Formula is buy 1 get 1 50%off online so hopefully it's also on sale at the store.
Buy 1 brush for $10.99
get another 1 for $5.49
Total = $15.49
Use $3.50 off coupon from www.ulta.com
Grand Total = $11.99 or $5.99 each
If it's not on sale at the store and you don't want to wait then you could still get a brush for $10.99 and use the $3.50 off coupon making it $7.49 which is still an awesome price.