Is there a simple way to create a javascript lookup table?

You can index prices in a two dimensional map on page load (with working fiddle).

1) I put the select values in lookup-tables in case you have to preload them:

var tables = {
    Colour: ["Blue", "Red"],
    Size: ["Small", "Medium", "Large"]
};

2) Here is your price table in array form:

var array = [
    {Colour: "Blue", Size: "Small", Price: 45},
    {Colour: "Blue", Size: "Medium", Price: 48},
    {Colour: "Blue", Size: "Large", Price: 98},
    {Colour: "Red", Size: "Small", Price: 65},
    {Colour: "Red", Size: "Large", Price: 31}
];

3) Initializing selects (populating values and event ‘change’):

for (var key in tables)
    if (tables.hasOwnProperty(key)) {
        selects[key] = form[key];
        selects[key].addEventListener("change", updateSpan);

        var values = tables[key];
        len = values.length;
        for (i = 0; i < len; i++) {
            var option = document.createElement('option');
            option.appendChild(document.createTextNode(values[i]));
            form[key].appendChild(option);
        }
    }

4) Indexing your price table:

len = array.length;
for (i = 0; i < len; i++) {
    var record = array[i];

    if (typeof map[record.Colour] === 'undefined')
        map[record.Colour] = {};

    map[record.Colour][record.Size] = record.Price;
}

5) Function updateSpan (on select change):

function updateSpan() {
    var Colour = selects.Colour.options[selects.Colour.selectedIndex].value;
    var Size = selects.Size.options[selects.Size.selectedIndex].value;

    if (typeof map[Colour] !== 'undefined' && typeof map[Colour][Size] !== 'undefined')
        span.textContent = map[Colour][Size];
    else
        span.textContent = "Price not defined to Colour: " + Colour + " and Size: " + Size + ".";
}

6) Debugging (hit F12 in Chrome or Firefox to open Console View).

Full indexed table:

console.log(map);

Just the price of ‘Blue’ & ‘Small’:

console.log(map['Blue']['Small']); // outputs the value: 45