^^^
At that price, you can probably get a boost inverter using an inductor or flyback circuitry from the usual Chinese sources.
The charge pump circuit charges electrolytic capacitors in parallel then switches them in series to provide a boosted output. You get noise from the inrush current of charging the capacitor and noise from the transfer of charge from the main supply and switching noise and charge pump current capacity is small. The next step up is the flyback converter and you have some things in your favour there. When the inductor is switched on, the current ramps up instead of switching suddenly, so one source of noise is reduced and the best way to reduce noise is to not generate it in the first place. There is still noise when the inductor is switched off, but current capacity can be larger and efficiency is higher since switching on occurs at zero current. Feedback is used to stabilize the output whereas a charge pump really doesn't do that. You can get the switching frequency to be above the audible level with a number of designs. There are only a few charge pumps with inaudible switching and as you say, 25 euros is a lot of money for an inefficient design.
