Replacement of a driver in a speaker is not simple. The crossover is designed to work correctly with that specific driver, and it will not work as intended with another driver unless you get lucky. Sometimes you do get lucky, but you're trying to avoid spending $100. Maybe you'll spend $50 instead, and save $50, but you'll likely end up with something that doesn't sound right and you'll end up spending the $100 in addition to the $50 that you already spent.
The manual that hex168 found provides some help. The sensitivity of the speaker overall is 89 dB, but this will not be the sensitivity of the midrange driver per se, for a couple of reasons. First, the tweeter and woofer both contribute to midrange output. The amount they contribute depends on their natural rolloff and on the crossover slopes. Any guess as to what it might would be foolish, beyond guessing that it is most likely not more than 5 dB combined for both drivers. The other reason is that the driver is padded, as revealed by that online manual. There is a resistor shunting the driver (10 ohms) and another in series with the parallel combination (4.7 ohms). The midrange driver was more sensitivity than it needed to be, which is common, and it was padded to lower its sensitivity, which is also common practice. Unless the replacement driver you choose happens to have sensitivity very close to that of the original driver, and also impedance very close to the original driver, you'll likely need to change the values of those two resistors. The crossover for the midrange consists of a single inductor and single capacitance (two in parallel is essentially one) in series with the resistor/driver network. You may get away with not changing these components, but it might sound funky if they are not changed such that the original high-frequency and low-frequency roll-offs are preserved. To do all of this correctly, you're basically designing the crossover filters for the midrange from scratch.
If I really wanted to preserve this speaker and if I thought that the driver you found is engineered expressly to be a drop-in replacement for the original, I would spend the $100 for it. In fact, I'd consider getting two of them and replacing the midrange in the other speaker at the same time. The reason is that even if the speaker doesn't end up sounding bad it will still sound different, and may even sound better than the original, but if it does, you still won't like it if the two speakers don't sound the same.
In short, the thing you are wanting to do is something that only people experienced at crossover design can really do well. For the rest of us, you cross your fingers and hope to find a true drop-in replacement and prepare yourself to pay whatever you have to pay for it. Otherwise, you put the speakers in the garage or the attic and then wait for another ten or twenty years before you can bring yourself to part with them.
