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[Monster] Strongholf of Player Safety Monsters [Reccomended combat level] Depending on monster: 20-70+ respectively [Details] 
The cockroaches of the Stronghold of Player Safety drop a varienty of items worth several thousand gold each. The Higher level one's can drop rare and expensive rune item's and are therefore a decent way to earn money.
Money making guide - The F2P RuneScape Wiki
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