Normal boosts at $0.99 I feel that it is more than reasonable, and to quote the film Lock, Stock and 2 Smoking Barrels “its a deal, its a steal, its sale of the f**king century”
I can surmise of 2 possible reasons why it is not so popular.
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It has a 1000 kill shelf life, once its gone, its gone, where as auto map reveal, or a pet lasts you a life time.
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It offers nothing that you cant do or get in game already, again using auto map reveal, no amount of game play, or monsters killed will it ever drop from a monster, the ONLY option to get auto map reveal is pay for it, and again you gave us that option for a super low price, personally I would have paid a lot more for a feature like that, $10 seems fair to me, especially since its a one off purchase, and more to the point I would be happy to pay for it, not because I am forced to pay for it to progress in my game, but simply for my own personal ease of use, and of course to support the game and devs.
A suggestion or at least something for you to mull over, have you guys thought about a subscription service, im not talking like WoW, pay monthly for £15, but smaller nominations. I have played other games where a bi-monthly subscription has been £3, and for that money you got access to a weapon that was no way near over powered, but slightly ever so better than others, or you had access to different maps, or even an increased drop rate for that month. No reward was ever game changing, or made you OP, but you did get tiny little perks for for your contributions.
Perhaps im over stepping the boundaries now with my next suggestion, so please forgive me if you feel that I have, after all it is only my humble opinion.
Instead of $1 = 1000 kills, could you apply a set time to the boost ? perhaps $1=1 hour ?
I know the multi boost is $10 per 2500 kills, but how about $10 for a month long multi boost ?
Personally I feel the way to generate income to support the game, is via IAP’s, but not to make it pay-to-win or pay-to-play, but to provide something that is impossible to get in the game, like wings we have now, totally cosmetic, it does nothing for your game play except makes you look different from everyone else.
Would I pay $10 for a Jester skin ? damn right I would 
Would I pay $5 for a pet skin that looks like a Jester ? here, take my wallet and take out the money yourself 
Just as im certain, anyone else with a screen name that could be married up with an object or person skin for their character would do the same.
Back on topic about the big multi boost, $10 is very reasonable, not for the reward you get, but for a donation towards the game itself, the multi boost is a by product of you supporting the game you love. I play DQ, and I play WoW, I love both games, I pay Blizz my £15 a month and enjoy my time in game, you guys give me a game I love and I enjoy just as much, but you dont ask for any money in return, perhaps everyone should buy the multiboost at least once in their playing career of DQ, if for nothing else other than supporting the game that has given them so much fun and enjoyment. Games like Torchlight and even D3 you give payment initially for the product when you first purchase it.
Again forgive me if I overstep any boundaries …
$10 for 2500 kills might seem high to some players, as those kills would take approximately 1 hour to use up, yes you get 100% increase in luck, gold, CS and MS, but if you played the game for 2 or 3 hours, you could collect more gold, CS and MS and even items but for free, costing you nothing but time. Perhaps throw in something that can be kept for ever, like a unique pet skin ? or even a guaranteed legendary pet ? or something along the lines that is unique to that purchase that cant be gained in game.
