In my answer, I am assuming that you have a table from which you can look up or reference Market values for each asset specified in one of your asset fields.
I suggest that you first modify the design of your tables and relationships as that will then make what you want to do easier to set up and is an approach that also makes for more flexibility.
Set up a related table to use in a portal in place of the fields asset1 through asset5 and marketvalue1 throughMarketValue5. Create one record in this table for each pair of asset and market value fields in your current version. This then enables you to set up these relationships:
OriginalTable---<Asset_MarketValue>-----Assets(market values to be looked up are stored here)
OriginalTable::__pkOriginalTableID = Asset_MarketValue::_fkOriginalTableID
Assets::__pkAssetID = Asset_MarketValue::_fkAssetID
See the first post of this thread for an explanation of the notation that I am using: Common Forum Relationship and Field Notations Explained
WIth these relationships in place the MarketValues from Assets can be either copied from Assets into the matching field in Asset_MarketValue or you can add the market value field from Assets directly to the portal row. The first option captures and preserves the current value and does not change if the value from which the value is copied later changes. The second option automatically update to show the current value each time the value in the related record in Assets is modified.