Inexpensive solid-state variable resistor
If you want something that behaves more like a resistor, you can use a photocell and light it with an LED from a filtered PWM.  That's acting as a 2-terminal variable resistor rather than a 3-terminal pot, though.

You could control all the LEDs from a single microcontroller using something like the TLC5940, which has 16 PWM LED driver outputs, with brightness of each programmable over a serial connection.  You'd need 10 of these at $1.84 each to control 150 channels, though twice that if you need two resistors per channel (to simulate an actual pot).



Also, have you looked at ICs with lots of pots inside?  $0.33 per pot is better than $1, for instance: 


DS3930 Hex Nonvolatile Potentiometer with I/O and Memory ($1.95 @1k)
AD5206:  6-Channel, 256-Position Digital Potentiometer (Price 1000 pcs. $1.94)


You could also look into voltage-controlled or programmable gain amplifier ICs, which might take the place of both an op-amp and a pot:


SSM2164: Low Cost Quad Voltage Controlled Amplifier - 4 channels for $3.12
MCP6S28: Single-Ended, Rail-to-Rail I/O, Low Gain PGA - 8 channels for $1.85


As for a computer-controlled many-channel graphic EQ, a DSP is a cheaper option.  For instance, TI, AKM, and Analog have audio signal processors with ADCs and DACs built-in, and easy to use GUIs for making the EQ, though you need to buy the development board. :)

Have you seen Digitally-Controllable Audio Filters and Equalizers?