So, if you wanna get paid £50 an hour, which seems reasonable, that budget gives you 100 hours to get that done.
Let's ignore all the paths, and the greenery for now, and just look at the 18 houses and 2 massive buildings. Just for sh1ts and giggles let's pretend we're doing 20 houses, which leaves 5 hours tops to finish each one.
That may be enough for the smaller, less complex ones, but I'd bet it's not for the big ones, or even the more complex smaller models. And that's just the modelling. If they want things exact dimensions as well, that takes even longer !
And all those hedges might take quite some time to get looking just as they want, and by the time you've factored in trees, and sky, and lighting, and placement, and roads, and plots, we're running to about +50 or more hours over the 100 we allowed ourselves while we were trying to get paid properly :)
So working at roughly that sort of scale, if you have to put in 200 hours to get this done, then you'd be on £25 / hour, and less if it takes longer.
I have done jobs for £25 an hour, but it's not ideally what I'm looking for, and £5000 for all that does seem at the bare limits of feasibility if the buildings are as detailed as you suggest.. I certainly couldn't model and texture all that in 100 hours. But then I am quite slow :) I'm sure there are speed-freak modellers out there who would tear through this, and perhaps it's one of those you're bidding against.
I am working on a long term Film Foundation project at the moment for a record low of £10 per hour, but I've chosen to do that because of the artistic integrity of the project, and because it's all of the sort of work I most like, and none of the stuff I don't, and doesn't get in the way of better paid work. So I guess you just have to weigh each job on its merits and how much of PITA' it is !