Perhaps a very effective defensive system/weapon that is very, very expensive. Expensive as in the range of hundreds of thousands or even a million.

Would give traders/miners a reason to accumulate credits.

What is currently the "Gatling Turret" used to be upwards of twenty-six thousand credits during the Alpha test. The Valkyrie, Prometheus, and Marauder were each thirty thousand credits (without going to the black market in sector 18), and it would be safe to assume that had the behemoth been around then it would have been even more expensive.

Of course, back then, weapons and ships had fixed prices from station to station. only cargo had a dynamic economy.

I agree that credits should be made more valuable, but I don't think a defense system would work too well. Either it would be too effective, making it so miners/traders never lost ships anymore, and would also only need to buy 1 defense system, which means it would actually make less things for traders/miners to spend money on. Or else it wouldn't be effective enough, and traders/miners wouldn't bother buying it, because there'd still be a high enough chance that they'd get killed and lose it, and you have to pay for a new one.

What I think would maybe be better way to make credits more valuable, would the optional subscriptions like ctishman suggested: http://www.vendetta-online.com/x/msgboard/3/9402