- A lot (or more than they're paid at the moment)
- Nothing at all
The rationale behind the first is that MPs need to be effectively unbribable by outside interests, either via the traditional envelope of used fivers or sinecures and stipends as "non-executive directors" after they leave office.
The rationale behind the second is that MPs must do what they do out of a sense of duty to their country, rather than the mere taste of silver.
There is an inherent conflict between these two extremes. Both purport to ensure that only people of integrity are elected to (or allowed to remain in) high office: but this objective is not served by the current mess of low salaries (and yes, for the job they do they are low) supplemented by perks and expenses.
My preference is for a situation where MPs are paid nothing, but must create some kind of external income to enable them to carry out their political duties.
This would ensure that MPs are canny, experienced, and entrepreneurial enough not be total dishrags in office, and (assuming that everything is declared in as open and honest way as possible) it would also allow us (the electorate) to see who works for who in as straightforward a fashion as possible.