Linear Interpolation Headache

I've been searching in vain for a solution to a relatively simple bit of mathematics that I’d like to Access to undertake.

It involves interpolating between data, as illustrated in the following example. Basically have several items, and let's say different "prices" dependent on the number of units purchased. The relationship between the “prices” is not a straightforward formula, therefore I need a query to return an interpolated price for a given number of "units".

DATA
ItemID Price10Price20Price35Price50Price100Price 1000
1£70£135£200£315£800£6500
2£23£31£61£89£210£1500

OUTPUT
ItemIDUnitsPrice(int)
117?
231?

So, for example
if Units <10, Then Price(int) = Price10
if Units = 17, Price(int) = Price10 + (17-10)*(Price20-Price10)/(20/10)
if Units = 32, Price(int) = Price20 + (32-20)*(Price35-Price20)/(35/20)
if Units >1000, Then Price(int) = Price1000

I'm aware that IIF statements might do the job, but I have 15 different 'Price' points, and don't particularly want to nest them all.

This problem was solved previously in Excel using a clumsy set of VLOOKUP/HLOOKUP functions in conjunction with If statements.