[{price:"150,00,00 gp"/** 15 gold pieces*/},{price:"4 sp"/** 0.4 gold pieces*/}]

db.collection.aggregate([{"$addFields":{"newField":{"$split":["$price",","]}}},{"$project":{price:{$reduce:{input:"$newField",initialValue:"",in:{$concat:["$$value","$$this"]}}}}},{"$addFields":{"rate":{"$switch":{"branches":[{"case":{"$regexMatch":{"input":"$price","regex":".*gp*."}},"then":"1"},{"case":{"$regexMatch":{"input":"$price","regex":".*sp*."}},"then":"10"}],default:1}}}},{"$project":{price:1,universalPrice:{"$multiply":[{"$toInt":"$rate"},{"$toInt":{$first:{"$split":["$price"," "]}}}]}}}])