I’ve done some online searching and found a way to get the full set for £26! 
On Wordery (online book store) there are about 4-5 pages of different versions of the Chronicles of Barsetshire!!! After searching about 5 online book stores, I have found that the cheapest set appears to be on Wordery.
All 6 books are paperback. The only thing is that I have found the first 3 novels in one collection while the other 3 novels would have to be purchased individually (unless you manage to find a set for that too!! I have had no luck!).
Ok here goes...
Anthony Trollope, Chronicles of Barsetshire Books 1 23: The Warden, Barchester Towers, Doctor Thorne (Masterpiece Collection) is £16.72.
Then the other 3 can be purchased individually: Framley Parsonage (£6.67), The Small House at Allington (£7.23), The Last Chronicle of Barset (£8.20).
The total cost would be £38.82 including delivery.
However, I think I have found a way to cut approx £12.50 on this price by doing the following:
(Bare with me!!!)
Have you heard of Quidco before? Its basically a site that you can become a member of for free and which gives you discounts for online purchases at about 4000 different retailers/merchants. It gives discounts at various book stores including Wordery. This is how it works: When you want to shop online you login to your Quidco account and then go to the page for the retailer you want to shop at - so in this case you would find the page for Wordery on Quidco. Then you would click on Wordery from Quidco which would take you to the Wordery website. Wordery pays Quidco for “referring you” - it pays Quidco about 5.5% of your total purchase then Quidco passes that amount onto you! So on £38.82 you would get about £2.13 back. Quidco makes its money via adverts etc - the more members it has, the more money it can make from adverts - so its win-win. At the moment, Quidco is running an offer (I don’t know if this offer is open to all members or only certain members) - whereby if you spend at least £25 at certain book retailers you get a £10 bonus from Quidco on top of the cashback of 5.5% of your spend. Hence for your purchase of £38.82 you could get a discount of £12.13. Even if you are not entitled to the £10 bonus offer you would still get a £2 discount on the price plus Quidco runs lots of different offers at different times of the year.
The total price would be £38.82 - £12.13 = £26.69 for the full set of novels.
PS. There is another site like Quidco - it’s competitor - which offers the same kind of deals: Topcashback (its free, there are no catches & it takes 30 seconds to join - just enter your email address & choose a password). If you join before 12th December you get a free £5 Amazon gift card. After joining get access to further discounts on deals & Christmas shopping at 1000s of retailers.
I hope this helps!!! 
Now i'm going for a 
and a 