custom development should never be the first choice. $500 off the shelf software could save you from spending $50000 on custom software plus added benefit of support and future upgrades (at a cost of course). Not to mention it would have been tested better.
If you do not find a software that addresses your needs, than it is a different story. Even than, custom developed part should be kept at minimum at possible by using ready commercial and free open source components.
If producer kills the product or goes out of business whatsoever, you're
grounded. You may even be unable to migrate data to other system or migration
could be very, very painful.
You most probably won't be able to integrate it with any of your systems
except maybe for big, very expensive ones that happen to be popular.
> Not to mention it would have been tested better.
Except when it wouldn't. I've seen such cases, and they are not that rare when
it comes to systems that are not general purpose.
To all this add the fact that by buying a system you totally loose the control
over bugs in it. You may be hit by a subtle bug that is a show-stopper for you
and the producer may refuse to fix it, claiming it to be low-impact.
If you do not find a software that addresses your needs, than it is a different story. Even than, custom developed part should be kept at minimum at possible by using ready commercial and free open source components.