Not quite so good in the UK, I think ((in part reply/response to martinuzz, this is)), though there are limits on campaign funding (seperately local and national, and there have been penalties for mistepresenting spending on one as the other) and Party Political Broadcasts are by a fixed allocation of free slots on the various national broadcasters' channels, so minor-but-significant parties neither get priced out of the market nor so easily wipe the floor with everyone else by having a Sugar Daddy funder dominate the field. (To grossly influence UK politics by media, your best bet is to become owner of a newspaper group and then lean on the respective editors if they aren't already going your way.)
Administrative costs are paid (strictly?) from public money, campaign funds must still be raised per-party, but donations (>£7500, per donor?) must be declared. Including 'loans'. Membership fees (individually below that radar), patronage, organisational affiliation fees, etc are accountable for to various degrees, within the above circumstances.
Past issues with Cash For Questions/Honours/Influence have meant tighter rules (often with fall-out, politically, at the time) though how well they have restricted Cash-For- behaviour in government/opposition circles is dubiously calculated. Cash For Contact (buy the opportunity to have an afternoon playing tennis with the Chancellor of the Exchequer?) seems to still be a thing, but then again that's minor compared with the rest of the usual Lobbying that's done in all forms both in and out of the actual lobby of the Houses Of Parliament.
The most recent issue (ignoring Partygate, which was nothing to do with this - probably!) is second-jobs by politicians (esp. ministers). Generally it's considered good to have an elected official with real-world experience, possibly ongoing if they can still do their jobs as MPs, MSPs, AMs, councillers, whatever. Those with medical experience who have (re)joined work in their local hospital/surgery to help with the overload caused by the pandemic are applauded. Those with legal training who continue to take caseloads on (often of internationally significant questions) less so. Doing an outside job with close relevence to their ministerial position (or a recent one) are rightly decried but I don't think the rules are yet in place to actually ban this practice, beyond a more general oversight via the Register Of Interests - for which there are already penalities for being 'economical with the actualité'. Being found to be on the board of a medical-equipment business (or a business that suddenly went into medical equipment in a big way!) and/or your wife is, and/or your friendly pub landlord coincidentally turns out to be, is frowned upon but I'm not sure if that's had time to be properly looked at yet.
We have annual Party Conferences, for the national (or sub-national) organisations concerned. There'll doubtless be localised rallies (somewhere between the former events and the cross-partisan local-election 'hustings' events, but I don't think they're anything like the US events in the stadia, or with the backdrop of airforce bases. Those will have to be funded under the allowed-funds mentioned at the top, I'd guess, though might include conservatively-valued donations 'in kind' by the site owner for nominal gross fees and thus zero net ones.
And of course there are press-conferences (often somewhere you have to wear a hardhat and hi-viz, if not actually fumblingly get onto the controls of some handy work-vehicle to knock over a symbolical wall of coloured cardboard boxes to represent your current policy target or opponent) but not sure what happens there except that you have to count upon the news crews wanting to cover it, in order to not waste everybody's time more than you already have (by shutting down a large part of the site to set up any stunt, arranging a tour of anything worth being seen looking at and release willing employees from their shifts long enough for some 'with the people' photoshoots). Some stunts seem easier to do (be see in a good old graditional British pub holding a half-downed pint of gold old British bitter) and others might need a lot more arranging.
As a public, I don't think we generally accept that we're all that impressed by big-money attempts to get messages out to either loyal followers or the fringes of non-followers that might be swayed (media-led messaging, especially in echo-chamber-type appreciation of a given outlet, seems to be the big driver) but I imagine such moneys as are spent are generally pushed in directions that are fairly optimal and may hide a huge overt spending but still are proportionally effective compared to the actual costs.