How much should I expect to pay if I want a usable road bike?
The cheapest we've seen that looks like it might do the job and be fun to ride is the Decathlon B'Twin Sport 2 for £229. Decathon's bikes tend to be good value for money and pretty well sorted. Evans do the Dawes Giro for the same money.http://www.decathlon.co.uk/EN/sport-2-16v-42393911/http://www.evanscycles.com/categories/complete-bikes/road-bikes?page=1
Going up a notch Specialized and Giant are two other brands that have always really punched their weight at the entry level delivering good bikes at excellent value for money. Expect to pay around £400 - £500 (the latter being closer to list). The Trek 1.2 at £450 looks pretty good too. At this time of the year it is also worth looking around for good deals on old stock.
You could also consider buying secondhand - your money goes a lot further then.
If you live within reasonable distance of a Decathlon I would go for theirs as they have a free 3 month service included in the price.
I had the entry one for a couple of years before selling it on and it is still going strong being used by someone as their commute bike.
It is never going to be the lightest bike in the world, but it will be reliable.
I bought a Decathlon Btwin 500SE as a winter bike. Having ridden it all winter I am very happy with it for the price. However, the rear cassette and chain were fscked after 1500 miles. Also the freewheel broke after about a month (Decathlon gave me a new wheel).
So components aren't as durable as a more expensive bike. If you do a decent amount of mileage you need to factor in the cost of repairs. I got mine in a sale for less than £300. Given the amount of use I have had out of it I think it was value for money. Hopefully, it will survive next winter without needing any major repairs.
What do you want to use it for? Fast club rides? Racing? Sportives? Commuting into town?
Most important thing at this level is to make sure the thing fits! Not just saddle height, which is adjustable enough but top tube length. You want to be able to sit comfortably on it without being too stretched out and too much weight on your hands.
Ordinarily a bike shop will size you properly but it can be pot luck in a decathlon or halfords as the staff member may be new or not fully trained.
If you can spend a bit more you can get some cracking deals on quality bikes that have been marked down from last year. There's BSOs (Bicycle Shaped Objects!) and theres serious road bikes. At this price point you can end up in either camp.
We've got a page on the best sub £500 bike offers. It will give you an idea of the sort of things you could get in your local bike shop (get measured up for your first road bike, don't buy the first one mail order unless you really know what you're doing - you'll want the after sales support from a good local bike shop)