REALLY BAD Lens Scratch

[timothetoad]

So being that this is a pretty bad scratch, it actually affects the outcome of my images...

What ways are there to fix this? And approximately how much would it cost just to replace that frontal glass?

Btw all those dust is apparently trapped under my lens filter...and I'm still in the process of trying to screw it off, but it's on waaaay too tight. I don't even remember tightening it up that much.

This is a Sigma 10-20mm F4-5.6 DC HSM Lens.

[Mt Spokane Photography]

You will have to get a estimate from Sigma, but its likely about $200, mostly labor. 
 
Normally, replacing the front element is not expensive, but it does depend on the lens design and large elements might be more expensive than small ones.  Some people have purchased just the element and replaced it themselves, again, it can be simple or very difficult depending on the design.

[RLPhoto]

It depends, probably not much for a sigma.

This is why I use UV filters.

[dukeofprunes]

Try not to use too much force when unscrewing the lens filter. You might be slightly bending it and thereby making it harder to get off. Be gentle and it might just unscrew easily:)

[Mt Spokane Photography]

Aluminum lens filter rings bind without the user doing anything wrong.  Brass is better, but more forgiving.
 
As a emergency fix, try using a wide rubber band around the filter.  That will give you a better grip.  Lens filter wrenches are very inexpensive, and a tool that everyone who uses filters should have.  You can get a pair for $7.95 that fit a 48-62mm filter, and a set for 62-77mm for 8.95 from Amazon.  I have adjustable ones from micro-tools that fit a wider range.
 
http://www.amazon.com/Camera-Filter-46mm-62mm-Filters-Package/dp/B0056483JK/ref=sr_1_3?ie=UTF8&qid=1340035714&sr=8-3&keywords=lens+filter+wrench

[timothetoad]

Wow thanks for the great responses. Never even knew there was a wrench for lens filters. Purchased both of them for my convenience!

I emailed Sigma customer support for the estimate; hoping the cost to not be as much as the lens itself. x(

[1982chris911]

What also usually works is if you use one of these iced thermal packs for camping or holding food cold in summer and let the lens rest on it with the filter ring having contact with the cold iced surface of the pack for about 5-10 min. Usually the filters goes off without any problem afterwards and you won't need any mechanical force ...