A mesh system depends on each of the points seeing each other (and having a discrete backhaul wireless channel)
They wont work through your shielded floor. You will need a wired gigabit cable from downstairs to upstairs as a very minimum.
Each Ubiguiti WAP cost about 70 quid, which is average for a good WAP. They are cabled to the router, so I assume its wireless router & 1 WAP down, and two WAP up stairs. Thats three cable runs and connecting ports.
400 quid is not unreasonable for a stable and reliable solution.
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Mesh wifi 
Mesh wifi 