That's why I changed them to be: +10 for $5, +20 for $15, and kept +40 for $40.
The multiplier is best for late game play, and not very good for early game. The main interest in the cheap item shop is for the early game, so we'd want something that does the opposite.
Acidic Maw is actually still pretty good. Let's look at in context compared to the STG. STG does 2 damage, salvo of 3, 2 energy per salvo, 33.3 energy per second consumption, and 100 DPS. Acid Maw does 1 damage, salvo of 1, 1 energy per shot, and 10 DPS. However, it also grants 20% damage.
For the Flame Tank (20 energy) you get 10 shots of the STG, 60 damage.
For the Blue Mech (100 energy) you get 50 shots of the STG, 300 damage.
For the Flame Tank (20 energy) you get 20 shots of the Acid Maw, 20 damage.
For the Blue Mech (100 energy) you get 100 shots of the Acid Maw, 100 damage.
So raw damage, it does under perform, but its advantage isn't displayed here. The advantage comes from the long term impact of that 20% additional damage bonus and how cheap it is to use.
At Tier 2 (the level you'd expect to get the Acid Maw), you will being using a main gun that does around 100 DPS, without any damage enhancements being included. With careful use of the Acid Maw, you'll gain an extra 20 DPS for 3 seconds, for however long it takes to kill the target because at 1 energy per shot, you shouldn't be running out very quickly. So you can have 100 DPS for 33 energy per second or 20% your total DPS for 0.33 energy per second.
Against regular enemies, it may not seem like much. It will reduce the time to kill it by 17%, which isn't going to be very noticeable on enemies that take 3 seconds to kill. But against a boss, it will cut your fight time by 17% (roughly). So that fight against Terminus, is a minute and a half shorter...
That's the kicker about the multiplicative bonus, it stacks and it is really impressive as the game goes on. Let's say you have a 100% damage boost consumable, now you would get an extra 40 DPS. How about late game when you have 3 offensive familiars, tons of damage boosts, improved crit... all of that is improved by 20%.
Let's consider boosting it to 100%. That means you are doing double damage. So you would effectively finish boss fights in HALF the time. Now, you might argue, but it is weak compared to other energy weapons. It is, if you only consider short engagements. When fighting regular enemies, energy weapons contribute damage quite nicely, because you get refills on room changes. When fighting a boss though, because the fight is so long, your energy weapon will typically run out without contributing very much. The Acid Maw is one of the few weapons that will endure, because of its cheap, long lasting effect. Now, one could argue that reapplying the debuff every 3 seconds is annoying, which... I have to agree with. This is why I proposed increasing the duration.