There is a weak sprocket in the timing chain system that typically starts making a very subtle noise and trips the check engine light before it gives up the ghost. When it does finally fail, what usually happens is the timing skips a tooth, and the engine stalls and won't re-start. If caught in time, the cost to replace all the chains, sprockets, guides, and tensioner is around $2500.
If the chain breaks completely, and the cams stop turning, whichever valves are left open will hit the piston in that cylinder as it continues to move up and down. The worst-case scenario is that one or more of the valves breaks off and ruins the piston, cylinder head, cylinder, or all three. Then... well, it's basically time for a fresh engine. We stock engines for Eurovans (usually around $5999.95), with a core deposit of $2500. If the engine is truly ruined, some or all of the deposit may be forfeited. So, a fresh engine installed can cost three times what it costs to do just the chains.