I tell them I can create a system of tables that allows each client to define their own set of custom fields, but of course that takes more time and money than "just adding a few columns".
I think you should push this option to your bosses since customizability is obviously a feature much in demand. Emphasize that an individually customized (rather than generalized, limited customizability) system for each client means that patches and updates will have to be created for each individual client (leading to longer roll-out times and higher costs); that non-standardized installations mean that HelpDesk tickets will take much longer to close (leading to dissatisfied clients and higher costs); etc.
In other words sell short term pain for long term gain by showing that the cost of their solution far outweighs the cost of your solution.
Salespeople are focussed on making the sale. That's what gets them their commission. They don't care about what comes after. Bosses, however, are focussed on cost. Sell to your bosses and your bosses can sell to the salespeople.
