Make it a vision of hell for your PCs.

One revision - I'd call the 'spell-sustaining increase' to be for 'over 1 hour', meaning you get the first hour as part of the spell, but get charged for it over that. And my math is wrong in there somewhere, because both of the 'Simstar' sustained spellcastings are much higher than I first wrote.
[ Magic + Spellcasting ] * [Drain DV] * [1 + 0.25 for every point spell Force is above Magic ] * 100. If the spell is an Essence-based spell, * [ 4 - (half Essence, rounded down to 1 decimal point) ]. For every quarter-hour the spell has to be sustained past the first hour, [1 + (Hrs / 4)] (first hour is part of the casting).
Standard Casting: (6 + 6) * (6 - 4) * 1 * 100 * [4 - (4.8/2)] * (12) = 12 * 2 * 100 * [4 - 2.4] * 12 = 2400 * 1.6 * 12 = 46,080¥.
Overcasting: (6 + 6) * (8 - 4) * [1 + ((8 - 6) / 4)] * 100 * [4 - (4.8/2)] * (12) = 12 * 4 * [1 + (2/4)] * 100 * [4 - 2.4] * 12 = 4800 * 1.5 * 1.6 * 12 = 138,240¥.
(If the first hour's 'free', the combat decker's cost drops to 31,680¥.) Note that an actual wage-mage probably wouldn't see all of this money, but the sim-star would be charged it; much of it would get taken by the corp, one way or the other, and after all, wage-mage gets paid the same amount no matter what he does. But it might give you an idea of how much a mage COULD make if he really wanted to put the screws to his clients.
Also, if the Force of the spell lowers the DV to less than 1, make that multiplier (2 ^ DV). A Force one greater than or equal to the reduction (4 or 5, above) would get you a '1'; one below would make it 1/2 (0.5), 2 below makes it 1/4, 3 below makes it 1/8, etc. The above cast at F1 and sustained for 12 hours would still be a 2880¥ charge.
You might also increase the cost for a sustained spell to x1.25 for that first hour, adding 0.25 to the multiplier for each quarter-hour after that. That'd make the prices:
Decker w/ Increased Reflexes: 35,640¥
Simstarlet w/ Prophylaxis: 47,040¥
Simstarlet w/ Overcast: 141,120¥