the ply might cost you £25 or thereabouts per 1220x2440mm board
bit more expensive if you buy 2440x610mm boards which are easier to carry
it is a wearisome DIY job but not complicated.
If you get a pro joiner or carpenter in, he will need all the carpets and furniture out of the way before he starts.
if you do it yourself you can take the time to remove underfloor rubble, clear airbricks, do any wiring or plumbing work, repair any damage, treat for woodworm etc which a pro will not want to spend time on.
why not try with a small room yourself and see how you get on?
you will need only simple tools. Claw hammer, bolster, jemmy or spade, saw, face mask, big vacuum cleaner to get the old one up.
drill, saw (pref circular), pencil, measure, straight-edge to fit the new.
here's one I did earlier.
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