I’ve been paying between £150-£200 pm for 7-11Mbps downstream, ~1 Mbps upstream. (Mind you, there were extras in that, such as traffic priority and things like email services, domain name hosting and registration x lots, that pushed it up a bit.)
If someone wants a guaranteed 900 Mbps d/s, then getting two FTTP links and binding them would be a lot cheaper than a leased line and you would get a guarded min 2 * 450 Mbps d/s. Mind you, you would also need to upgrade your whole lan, router, switch, ethernet cables to be able to cope with 2 * 900 Mbps, so 2.5 G ethernet would be a minimum, albeit not very future proof, and you would possibly be well advised to get all your LAN up to 10 Gbps ethernet plus an appropriate switch+router. Starts to become a serious up-front cost, and your hosts might not be able to cope with 2.5 Gbps or 10 Gbps, never mind the limitations of wi-fi.