veryone is familiar with the banks' plea: raising taxes on them amounts to taxing the nation's growth. If you increase how much banks pay, goes the argument, less interest will be paid on deposits and loans will cost slightly more, meaning we'll all be worse off eventually. In terms of economic theory, it's coherent, it should be said: in an ideal world, a chancellor would strive to lower costs of capital at every opportunity.

In the real world, however, John Healey faces multiple competing demands, even from within a business lobby that extends well beyond the banks. As this newspaper revealed last week, the chancellor is likely to increase bank taxes in his budget next week. In the current climate of trying to "stay the course" on the fiscal rules by finding some money from somewhere, tapping the banks is a reasonable policy. Or, at least, better than most alternatives.

First, today's conditions for banks are plainly gentle: defaults are historically low and interest rates are high. Banks' returns on tangible equity – a better guide to how they are doing than pure profitability – are heading towards 20%. Most non-financial firms would love such a number. Provided Healey doesn't go overboard, the flow of capital is not going to be seriously undermined when the starting point for banks' returns is so good.

Second, all the indications suggest Healey will indeed opt only for tweaks. If the mechanism is the surcharge that banks pay on top of corporation tax at 25%, then an increase to 8%, from 3%, would amount to £2bn-ish extra from across the sector.

He may not even go that far. Analysts at UBS calculate that a doubling of the surcharge to 6% would reduce UK banks' profits only by about 2% on average, with a roughly 3%-4% impact for the domestic-focused banks, meaning NatWest and Lloyds. That cannot be called a violent change in the operating climate, or a devastating blow for the shareholders or the equity-incentivised executives. Dividends or share buybacks may be trimmed a bit, but the share prices for both banks are up by a fifth in the past year.

Third, we'll have to wait to see whether any move by Healey is part of a wider financial services package that could include bank-pleasing elements. There's always scope for another hack at the thicket of regulations, for instance. Or help on capital buffers. And, if Healey is really as pro-growth as he claims, he'd consider this column's perennial pre-budget suggestion to scrap or cut stamp duty on shares – a move that would seriously improve UK companies' cost of capital (see Tax Policy Associates' paper) and do wonders for banks' wealth-management divisions.

The point is that a small turn on the surcharge dial is better than other ways of extracting the odd billion or two for the banks. A windfall tax would be too arbitrary. Changing the way banks' deposits are remunerated at the Bank of England would look like interference in the central bank's independence. The surcharge is a well-understood mechanism.

None of which, one suspects, will stop the howls of complaints from banks, or halt the grumbles about the loss of competitiveness versus other financial centres. But an extra £2bn, or less, from the financial services sector simply doesn't sound like a gamechanger if Healey acts as expected. Even JP Morgan's Jamie Dimon, who likes to play politics at budget-time with his proposed new London headquarters, might reflect that some other European capitals – Paris, say – don't look like a bed of roses.