Im guessing around 15-25m , depending if your going fast or slow and what logs you burn :')

it would cost you around 5 mill rsgp using maple logs on a bonfire
you would need to buy 100000 logs probably less cause when you hit level 75fm exp rates go up for maple logs
i dont know how long it would take you as i dont know how long you play each day but it would most probably be less than 5 days if that

U can do it free for 300-400k exp per hour. look it up. you cut these roots and burn them

Pixels wrote I need 99 on firemaking and its one of my low stats so dont judge me XD
its only at 63. Im fine with spending up to 4-5 mil. But to people who already have it how much is it going to cost me?/how long to light all the logs?

man best thing to do is get your woodcutting up to level 83 and firmaking to 83 then got to curly roots and cut and burn them you get 4 roots a cut and when you burn them you get 379xp you can get to 150xp to 200xp a hr hope this helps

Firemaking is a very easy skill to train, as it doesn't require much work, and is afk-able, while giving high amounts of xp. A very good suggestion is burning Jadinko Roots, as some have listed above, as they can easily give 200k-300k exp p/hr. Although it also requires 83 woodcutting, its very worth while for the free, high rate of xp. You can find guides for them on YouTube.
Really the only other option is burning logs regularly or in bonfires. To find the amount it will cost for 99 plain burning/bonfire, take the xp you have left until level 99, divide it by the amount of xp given by a single log of the type your burning, (giving you the amount of logs needed for for 99) then multiply that number by the cost a single log to find the cost for 99. Hope this helps! And the best of luck to you!

well you arnt even close to being half way to 99 sorry to break it to you. i think lvl 93 is halfway to 99. but ya you should spend up to 4-5m on maples thats what i did i think and you lvl pretty fast and its better then buying yews.