With the examples in question, it must be stressed that we're dealing with extremes. On the one hand, Wetherspoons by some distance sell the cheapest beer in town and Ruddles Bitter itself is much cheaper than their guest beers (£2.15 a pint). On the other, £1.95 for a half is well above what you'd pay in most city centre pubs, tenanted or otherwise, where £3.30 is about the average price for a standard bitter. Even so, how can 'Spoons charge so much less and still turn a profit?
Well, of course, as a chain of nearly 800 megapubs they have huge buying power and can negotiate keen prices with suppliers (though micro brewers I've spoken to assure me they pay a fair price, taking volume into account). Also because their pubs are so big and throughput so great they can cope with a small per-pint profit. Finally, the pubs are managed so they have central control over most aspects of the operation.
Tenants of breweries and pub companies have to buy their ales through their landlord, paying significantly more than the "free trade" price as a result. As owners of their own business they must also meet all the overheads from rent and rates through to utilities and staff costs. The current turnover of tenants is proof enough that running a pub is no easy recipe for riches - many are struggling just to get by.
But - it's a free market out there and you can't blame Wetherspoons for adopting their (successful) business model. As with supermarkets, we customers who live or work in Cambridge can choose where we spend our money (it's different out in the villages of course). From my experience what you get in the Regal (and hopefully the new Tivoli too) is a wide range of well-kept and interesting real ales at low prices. What you don't get is the atmosphere of a small-scale, community-focused traditional British pub whose licensee has a close relationship with his or her regulars. Many of us feel that such an experience is well worth paying more per pint for - but for others it's not a consideration and that's their entitlement.
And having said all that, I'll leave it to you, the reader, to decide if £1.95 is a reasonable price to charge for a 3.7% beer, even in a tenanted pub.