The gap between the best enhanced annuity rates and the lowest standard annuity rates stands at 30%, according to the latest Annuity Index from retirement specialist MGM Advantage.
Someone with a shortened life expectancy (because of a heart condition, for example) and a £50,000 pension pot could buy an enhanced income for life worth as much as £3,538 a year. The same amount used to buy one of the poorest-paying standard annuities would provide an income of just £2,722 - an annual difference of more than £700.
Over the course of an average retirement that difference in rates equates to £17,000 in lost income.
For the first time, the MGM Advantage index shows rates for standard and enhanced annuities moving in different directions, as the former rose by 1.6% over the second quarter of 2014, while the latter fell by 0.4%.
More generally, there is considerable uncertainty over the future direction of annuity rates at present.
Aston Goodey, marketing director at MGM Advantage, points out that there are conflicting forces at work. "The market has already felt some pretty hefty punches (for instance increasing longevity, growth of the enhanced market), most of which meant downward pressure on rates.
"The light at the end of the tunnel was the positive impact of future increases in interest rates, and their subsequent impact on gilt yields, which should help to push rates up."
Against this, he speculates that the Budget changes to the pensions regime are likely to reduce demand, pushing prices upwards. Moreover, future annuitants may be those with longer life expectancy who want a secure income – again putting downward pressure on rates.
"Although we haven't felt the full effect of the changing market dynamics, annuity rates are likely to flatline for a while yet," he adds.
In the meantime, the sums involved in these differentials underline the continuing importance of comparing prices on the open market before buying.
This is a message further emphasised by a study of retirees with pension pots of between £25,000 and £75,000 by Just Retirement. It finds that seven out of 10 pension savers who took the annuity offered by their pension company without shopping around "would do things differently" if they were in that position again.
The study finds that of those who would do things differently, 70% would take professional advice, 43% would compare prices themselves, and 14% would leave their pension fund invested.
Those who took their pension provider's offer without shopping around did so because they expected the provider would offer competitive rates (40%), or because it was the easiest option (30%). Three in ten did not even know they could look elsewhere.
Stephen Lowe, director of group external affairs at Just Retirement, says: "Clearly many of them now recognise they could have done better elsewhere. For example, eight out of 10 said there was no mention during the buying process of enhanced plans that could have delivered a higher income due to medical conditions and lifestyle factors."
Earlier this year, the Financial Conduct Authority found that 80% of those who buy their retirement income from their existing pension provider could have done better if they'd searched in the open market.
Lowe flags up the influential role of providers in ensuring consumers get a good deal.
"The government has promised guaranteed guidance next year for all pension savers heading into retirement. There needs to be just as much effort on controlling information and communication between providers and their clients to ensure people are not being led into solutions that are not in their best interests," he says.
This article was written for our sister website Money Observer