Power Grid Resource Costs

Python 3, 71 69 bytes

Thanks to @xnor for -2 bytes

f=lambda r,a,b:b and[8-int(~-a/3),max(18-2*a,13-a)][r>2]+f(r,a-1,b-1)

A function that takes input via argument of the zero-indexed resource type r, the amount available a and the amount to buy b, and returns the cost.

This makes use of the fact that True and False equate to 1 and 0 in Python, allowing the use of Boolean expressions to index into lists.

How it works

f=lambda r,a,b           Function with input resource type r, amount available a and amount
                         to buy b
b and...                 Base case: return 0 if b=0
[8-int(~-a/3),...][r>2]  If not uranium, yield the unit cost 8-floor((a-1)/3)...
max(18-2*a,13-a)         ..else yield the current uranium unit cost
...f(r,a-1,b-1)          Decrement a and b, then pass to function
...+...                  Add the cost of each unit to give the total cost
:...                     Return the above

Try it on Ideone

Javascript (ES6), 71 59 bytes

f=(t,m,b)=>b&&(t>2?m>4?13-m:18-m*2:9+~(~-m/3))+f(t,m-1,b-1)

Takes type, market_amount and buy_amount as arguments. type is an integer between 0 and 3.

Demo

f=(t,m,b)=>b&&(t>2?m>4?13-m:18-m*2:9+~(~-m/3))+f(t,m-1,b-1)
function run() { var tests = document.getElementById("tests").value.split("\n"), test, C = 0, O = 1, G = 2, U = 3; console.clear(); for (var test of tests) { console.log(test); console.log("> " + eval(test)); } }

<textarea rows="10" id="tests">f(C, 24, 4)&#10;f(C, 20, 4)&#10;f(O, 3, 3)&#10;f(U, 1, 1)&#10;f(C, 1, 1)&#10;f(G, 0, 0)&#10;f(O, 10, 7)&#10;f(U, 12, 4)&#10;f(G, 11, 4)</textarea><br/><button onclick="run()">Run</button>

Befunge, 142 bytes

&2`#v_&&>:!#v_\:1-3/8\-v
v:&&<   ^-1\ -1p15+g15 <
v>#<v       <
! v5<
# 1:
>^g-
| 81
\ 4\
: *-
4 -1
` .p
# @^15+g15<
>:49+\-   ^
|
>:2*92*\- ^

Try it here! Takes input as 3 integers, where the resource type is 0,1,2,3. Output is an integer.

No idea if this can be golfed any better. There's not that much whitespace, but the newlines probably hurt.