Do I understand correctly that you asked for the alarm company to install pet resistant detectors but they installed DSC Encore 301D detectors? Now they want more money to install what you asked for in the first place? If that is the situation, they really should replace the detectors at no cost to you.
I notice you placed a "?" after the model number. If by that you meant that the existing detectors are actually model 301DP, then those are pet resistant. DSC claims they can handle a 60-pound dog.
I've rarely used DSC detectors on my jobs, partly because I had other brands that performed well and partly because a few DSC detectors I tried gave less than stellar performance. Everyone has his own preferences. I like Visonic's K-980D which is a dual technology (microwave plus PIR) pet resistant sensor. These require that both the uWave and PIR sensors be tripped to give an alarm, reducing the incidence of false triggers due to environmental conditions. This model does a good job of ignoring most pets though no motion detector can be guaranteed never to false.
There's also a less expensive, single technology model from Paradox called a DG75 which I've used with decent results as well. As I mentioned above, everyone has his own favorite models. These have worked for me and for several thousand DIY customers who have purchased systems from my online store. I carry a fairly wide assortment of competing brands. Although you are using a professional installer to service your system, you can get an idea what the installer's markup (plus labor) is by comparing the retail prices with what the technician quoted you.
It takes about five minutes to replace a motion detector and five or ten minutes to thoroughly test it once the swap is done. If your alarm company replaces three of them, you can usually expect them to charge something for the trip plus about an hour on your premises. If they made a mistake installing the wrong detectors at the start, they should fix the problem at no charge. If the detectors were something you specifically chose and they don't work right then it is within reason for them to charge you to replace them.
Regards, Robert L Bass