I use the old "cost to change" trick ie how much the old bike could be sold for. Careful pitching of low ball P2 helps as it is then but a baby step to what you really want. And then compare to other household items like eg new dining room table and chairs. Comparison with the cost of a really rubbish new car also helps, as does savings on rail card if said bike is to be used for such a chore as commuting. Of course it never is, unless it's sunny, mid summer, on a Tuesday, and you have no client meetings.
An example:
"Sweetpea, the man at cyclefit said my bike's the wrong size and will cripple me"
"So"
"Well, you know how I have all my suits made to measure as off the peg don't fit, it's the same with bikes. There's the safety aspect too."
"How so?"
"Imagine driving the car with the seat so far back you couldn't reach the pedals. It's the same with bikes."
"You have a new bike"
"I
had a new bike. It's old now."
"You bought it in 2007"
"It was a 2006 model. That's 3 years old. Most people change their cars more regularly"
<sighs> "Ok how much"
"Well about £2,500"
"For a bike???!!"
"No, just the frame. But it's only £2k if you take off my golf club membership".
"You don't play golf"
"No, but I could. And membership would be £500. Not just a one-off either, that's every year. So since my current bike was made in 2006, that's £2000 saved. So effectively the bike would be free."
"What about your current bike?"
"Well, I'd sell it of course, my sweet."
"How much for?"
"Ooh. Loads. Or I could keep the kit and wheels and just sell the frame, but a complete bike would go for more."
"Hmmm"
"I'd ride to work too, every day. That's £1,600 saved straight off, which would pay for the new wheels and kit. Probably"
"You said that last year. And still went in by train."
"It was raining"
"What all year?"
"Pretty much."
"Next door have a new Range Rover"
"Yes, and they dropped ten grand as soon as they drove out the showroom. So £4k for a bike is a steal"
"four thousand quid for an effing bike?"
"Well no. Only four hundred actually. two grand saved on golf club fees. 1600 saved on railcard, leaves just four hundred balance.
"
Job done.