I think increasing the price for bold, red, italic isn't the way to go. 5fg per is just fine i think. Especially for all newcomers with 0fg coming to the site trying to trade and earn their first forum gold.
Seen a few times people accidentally purchase some of the topic features and 5fg isn't to much big of a deal but 25,50,100fg would be imo. It's a grind from the start to get your first FG trading maybe gems and runes or some uniques.
However I was myself thinking about the filter and only seeing bold, red, italic topics. I like the idea but i can also see the negative side of it. If the price will stay at 5fg each it will be overused and having bold+red will be new standard and there will be only topics bold+red in the trading forums and maybe forcing people to spend fg on topic feature, don't think that way is the way to go on the site. Forcing people to spend fg.
Myself normally look for those bold/red topics in the trading forum because 9 out of 10 times its good items and if i dont maybe dont want to buy it myself its just fun to look at the item. But it can also be a opportunity to see what the item is and actually offer and buy it.
It can be a struggle to find certain items also using keywords especially if people making the topic is using a topic title such as "GG ITEM" and then just a picture inside and no keywords inside to fit your search filter.
I been struggling to find gold barbarian needs now since 2nd week into ladder and trying to find and optimize my character

I almost search daily for the words "extra gold" "gold find" "gold barb" and/or certain item names im looking for.
So i would enjoy the feature to filter for red/bold/italic titles at least so i voted yes on that part.
TL;DR Voted yes on feature seeing bold/red/italic titles