Random numbers to generate amount between two numbers

Hello,
I have to build an excel sheet to do an invoice.
I know the total amount and the quantities for the items in the invoice.
I have to generate random prices for them , but the total amount should be between 7000-7800, and if the quantity is bigger for an item, the price should be smaller.
Any ideeas?
Thanks.

Well, I can think of one simple way: your invoice in attachment has 12 lines; if it's always the case, you can assign the equal amount to each line, dividing total amount in C15 with 12, and then dividing it with quantity, so to get price for each item.\

In this example, in cell C3 you should type

=$C$15/(12*B3)

And then just copy it down in all range, C3 through C14.

If somebody inspects it, will see that amount quantity*price is equal for each row, so you may slightly change it, by not simply dividing total amount with 12, but assigning it different percentages of total amount, but to have 100% at the end; I mean, dividing by 12 means multiplying with 0.8333; instead, you may multiply each row with, let's say, 0.10, 0.07, 0.08, 0.05, 0.15, 0.06, 0.10, 0.17, 0.02, 0.09 and 0.11, which at the end sums up to the same total.

Also, if the number of lines is not always the same, then you may set additional cell, let's say D2, where you may enter number of lines for that specific case; then formula in cell C3 is

=$C$15/($D$2*B3)

But in this case it's not p[ossible to variate difference between lines like in nabove example, all lines will have the same subtotal amount.