"No need for replacement for 2 years When smoking five cigarettes a day" is probably a trick. Owning parrots and having to deal with the feather dust, I have two air purifiers that I have constantly running on low in my apartment. Every month I have to vacuum the filters as they are clogged with the dust they pull out of the air.
Yet, I feel like I'm breathing easier than two years ago and I don't really get a stuffy nose when I'm in my apartment anymore. And the same goes for my asthmatic father - he's not coughing anymore when he visits us.

Can't say anything about rm300+, but yes, gbmitoban, you'll have to change filters from time to time. Nothing against your decision, but what's the use in cleaning a single room when your furnace is spewing allergens all over the house? I'm using a Honeywell Drum purifier (filters need to be replaced each three months) and a smart one Winix HR1000 which is bigger (the manual states to clean the pre filter every 14 days, haven't replaced it yet, also I find it funny that it's called a "true HEPA" - like, then what's an untrue one?) so I'm running it in my living room. Both are HEPA. But you should keep in mind, that it's not an ultimate solution so to keep my allergies at bay I have to vacuum regularly.

Here are some nice sources:
https://acaai.org/allergies/allergy-treatment/air-filters - "Not changing the filter regularly can also do more harm than good". Please, keep it mind as well. There's no such thing as a everlasting filter that requires no maintenance.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3165134/ - here are some studies as well. Spoilers: "studies support multiple interventions, including air filtration, as methods to improve outcomes in the treatment of allergic respiratory diseases".
https://www.consumerreports.org/cro/air-pur...guide/index.htm - and here's a good basic maintenance guide.