There are several online vendors selling "5.4mm GoPro lens" (which are flat lenses removing the fish-eye look from your GoPro) and they are selling them for $375 or in that ballpark range.
First and foremost, these lenses (and the others of various focal lengths) aren't made for the GoPro. They are actually manufactured and advertised by the manufacturer as lenses for small security type CCD cameras. They do however fit nicely into the GoPro 3+ Black and Silver camera because the lens mount threading and size happen to be the same as is used in the CCD cameras they are meant for. These lenses do produce some nice, non fish-eye images when used with GoPro cameras. But if you are using the 5.4mm or higher focal length replacement lens on your GoPro and are flying it with a multicopter, make sure you have well balanced props, engines and so forth because this lens will magnify any jello effect on your footage from vibrations of the multicopter.
Secondly, if you do decide to purchase one, you dont have to pay $375 or any such nonsense for this lens. All these companies are getting them from china for around $100 and if you look around, you can find them from one US vendor for around $155 or so. A vendor making $55 profit per lens for the trouble of ordering them in bulk from china is definitely a fair price. A vendor making $275 or more profit on the same lens is a total rip off. There is nothing wrong with making a profit, but one of the most well known companies that are selling these for $375 is trying to strong arm or threaten anyone else that is trying to sell these cheaper because he doesn't want to lose his easy money train (or perhaps be found out how much he has ripped people off for) and that reason alone prompted this post. And don't let that company tell you these are special made for them cause they aren't. In fact, here is the 3rd link I found on page 1 just Googleing "5.4mm ccd lens" where you can see a couple Chinese companies advertising the exact same lens for CCD cameras: http://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=1%2F2.3+inch+F2.5+M12+10MP+CCTV+lens+5.4mm
Finally, if you do decide to get one of these somewhere that doesn't charge $375 for them, be warned that there are 2 versions of the 5.4mm lens and getting the correct lens DOES matter.
1)The "IR" version is meant for putting on CCDs filming at night where there is a infrared light shining in the area - this is the WRONG lens and you don't want it for daytime shooting. If you purchase this lens by mistake, you will get footage that is bleached pink like this:

2)The "IR CUT" version is the CORRECT version that you want for putting on a GoPro to film in the daytime.
See the sample photo below so you can tell the difference between them:

Good luck!
First and foremost, these lenses (and the others of various focal lengths) aren't made for the GoPro. They are actually manufactured and advertised by the manufacturer as lenses for small security type CCD cameras. They do however fit nicely into the GoPro 3+ Black and Silver camera because the lens mount threading and size happen to be the same as is used in the CCD cameras they are meant for. These lenses do produce some nice, non fish-eye images when used with GoPro cameras. But if you are using the 5.4mm or higher focal length replacement lens on your GoPro and are flying it with a multicopter, make sure you have well balanced props, engines and so forth because this lens will magnify any jello effect on your footage from vibrations of the multicopter.
Secondly, if you do decide to purchase one, you dont have to pay $375 or any such nonsense for this lens. All these companies are getting them from china for around $100 and if you look around, you can find them from one US vendor for around $155 or so. A vendor making $55 profit per lens for the trouble of ordering them in bulk from china is definitely a fair price. A vendor making $275 or more profit on the same lens is a total rip off. There is nothing wrong with making a profit, but one of the most well known companies that are selling these for $375 is trying to strong arm or threaten anyone else that is trying to sell these cheaper because he doesn't want to lose his easy money train (or perhaps be found out how much he has ripped people off for) and that reason alone prompted this post. And don't let that company tell you these are special made for them cause they aren't. In fact, here is the 3rd link I found on page 1 just Googleing "5.4mm ccd lens" where you can see a couple Chinese companies advertising the exact same lens for CCD cameras: http://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=1%2F2.3+inch+F2.5+M12+10MP+CCTV+lens+5.4mm
Finally, if you do decide to get one of these somewhere that doesn't charge $375 for them, be warned that there are 2 versions of the 5.4mm lens and getting the correct lens DOES matter.
1)The "IR" version is meant for putting on CCDs filming at night where there is a infrared light shining in the area - this is the WRONG lens and you don't want it for daytime shooting. If you purchase this lens by mistake, you will get footage that is bleached pink like this:

2)The "IR CUT" version is the CORRECT version that you want for putting on a GoPro to film in the daytime.
See the sample photo below so you can tell the difference between them:

Good luck!