The 'problem' with valuing most helicopters (let's exclude Robinsons as as different set of rules apply to them due to the 12 year thing) is that the value of the machine is aligned to a few different factors...a bit of age, a bit of upcoming maintenance, a bit of condition and a massive amount of component times remaining: but even this is further affected by the age of the components.
So you could have a pair of 1985 JetRangers side by side. One is tatty, has a lousy avionics fit, its annual is due tomorrow. Next to it is parked a machine that has just come out of Edmundsons, 2 off 8.33 radios, clean leather upholstery, sparkling windows. But the former will be worth more* as it has a freshly overhauled turbine, mid life blades and so on.
*to an experienced buyer. And therein lies the rub.
To answer your question 'how much do I need to spend', my answer would be 'as much as you can afford'. And it will start with a 3.