The costs, I think are primarily from bandwidth usage. Voxalot is based on asterisk, which does media handling. This can use excessive bandwidth and therefore rack up costs when there are many users. Sipsorcery does not do any media handling, only SIP signaling, so its bandwidth requirements are much lower.
Also you can check the voxalot servers' status here:
http://http://scopezoom.com/vox.php