I ran an E8 game for about a year.
I wouldn't bother changing the cost.
d20/D&D is based around something like 13 encounters and then you level. In theory, you hit level 20 after a year of playing a weekly game. I'm hazy on the specifics, but I'm sure there's plenty of people around that can give you the exact numbers if you want.
In giving out Feats, you're not significantly increasing the overall power of what a character can do; instead you're increasing the number of things they can do. Usually.
Having more options to resolve stuff (combat or non-combat) does increase the "power" of the character, but not the same way that a level bump of Saves, Hit Points, Ability Increases, and Class Abilities does.
People like to see change to their characters, and putting down a single feat every 5k really isn't a "change" for most people. Especially since many feats are "exception" based; meaning that they allow you to avoid being hit by a rule you were previously subject to. Like penalties for this or that attack, or Attacks of Opportunity, and so on.
Which means that while a character does gain some power, the feeling/perception of the _player_ is that not much has actually changed.
Also, Upper_Krust did some work regarding what various things like special abilities, Hit Dice, Feats, and so on were worth in terms of CR. As I recall (I don't have the document handy) and Feat is considered to be worth .2 CR. Yeah yeah, it doesn't take into account the deliberately designed-to-suck feats. It's still a good rule of thumb for the GM to take into consideration.
In other words, if you're worried about characters gaining "too much power" too quickly, I don't think it's going to be too much of an issue. Every 5 feats (25k of experience) consider the character to be roughly another level higher.
Obviously you have to be careful here. The overall CR system isn't fool-proof and using this rule of thumb isn't either. You want to make sure the monsters/NPCs are all following the same rules too. You shouldn't just toss in new higher CR monsters, just because the PCs now have more feats. Those higher CR monsters have additional stuff that's been packaged along with them that makes it a bit unfair.
The thing you're going to have to keep an eye out for is the... glass ninja?... thing. After a while, you'll wind up with characters that are _very_ competent, and can potentially deal some pretty serious smackdown in combat. But their overall ability to absorb punishment hasn't actually increased significantly. They can simply dish out more damage or have additional ways to resolve problems. If you simply increase the combat difficulty by using higher CR stuff, you run the risk of a cascade effect which can result in a TPK. Instead, you're better off increasing the number of opponents faced, or bumping up the toughness of some critters.
And of course, you need to do something other than combat.
Combat is the primary form of XP in d20/D&D, and chasing XP is the way that character power increases. However, running an E6/E8/E* game means you're _capping_ the overall power increase. Which means there's less incentive for players to chase the XP, since their characters will hit that cap; I've never played in a game that's managed to go from 1-20 in my 20 years of play, but getting up to 6th or 8th level? Plenty.
Since hitting the cap is going to be a very real and present thing for the players, you need to make sure they've got other stuff to do than just fight.
Think of it like this: Most people are into chasing the 1-20, zero to hero, only you can save the world kind of play. It's like a movie like Lord of the Rings or some crap like that. You start out running from rats, and eventually you're a superhero slaying gods and demons.
An E* game on the other hand is much more like a TV series. You've got characters that can do X. As the series goes on, sometimes they can do/learn some more stuff, but on the whole you've got a pretty good idea who and what they are.
I would personally suggest taking the cues for your game more from "Alias" than "Lord of the Rings". You can _do_ gritty combat and no magic (at least that's what a lot of people seem to be after with E6/E8 games), still have it be "heroic fantasy" (or whatever), and still be more "Alias" (TV series) than long movie.
A TV series tends to focus in on a relatively limited "base area" that the characters operate within, and then they deal with stuff. "Stuff" being: each other, random antagonists that show up, support characters (NPCs) and their relationship with the main Protagonists (PCs) and how that's affected by whatever it is the Protagonists are engaged in doing, and so on. Buffy, Angel, Alias, Lost, Dresden Files... all of 'em have some similarities in regards to this sort of thing. And not just those shows, but even something like House M.D. or Dead Like Me.
Running an E* game is a pretty simple thing on its surface. But there's a lot of subtle stuff going on in the background. From the GM perspective it starts off being "I want to run a low magic and gritty game" or more rarely "I don't feel like dealing with the pointless inflation" (my reason), but when you're taking away the player's toys like that, you need to think about what it leaves for the players to play with.
Keep the progression rate as it is, or make your life even simpler and just decide that roughly every 5 sessions, characters will have done enough to justify getting another Feat. It'll be a little bit of a feeding of the "improve the character" thing that most gamers are addicted to, and allow you to focus your game on more than just the XP/Level grind.
