The Five and Era 300 are two different classes of speaker. The Five is designed for music in stereo. The Era 300 is designed more for spatial audio music.

The Five is still Sonos’ flagship speaker for music and considered their “HiFi” speaker. The Five is also a larger speaker with deeper bass.

Check out these comparisons:

https://www.gearpatrol.com/tech/audio/a43358264/sonos-five-vs-era-300/

https://www.rtings.com/speaker/tools/compare/sonos-five-vs-sonos-era-300/15253/34837?usage=10159&threshold=0.10

There are some obvious differences. 300’s are smaller and weigh less. This reduces shipping costs all along the path from manufacturing to final shipment to the consumer. “Free shipping” is a marketing gimmick. If the purchase price doesn’t cover all costs, the enterprise eventually runs out of cash.
 

“Smaller” will be very popular with customers. I also suspect that the 300’s use of more modern technology results in some savings. Based on posts here in the Community, I suspect that 300’s are sold in pairs.

Finally, I think that it is expected that the 300’s will be more popular, selling more units, resulting in more economy of scale that can be passed through to the customer.

Within a current model line up, you can expect the prices fixed by the maker to reflect perceived value delivered. As a general rule of course, there will always be exceptions driven by marketing strategies where this may not apply.

Why is the dollar price of the Era 300 less expensive than the £ price

At current exchange rates the 300 should be around £360 not £449

Even if the priced it at £399 it would a resonable surcharge

Sadly the pound is not worth much anymore…also in the USA they have to pay a sales tax which in not included. The UK government then slaps on an import duty. 

As stated, all US prices are listed without taxes, because taxes are determined by individual states.  In the past, the list price sans taxes for US Sonos devices has been within a few pounds of the UK price with VAT and import duties removed.