Part I - Background
Part II - Comparissons and respective worth of 30 solidi broadly

Part I

I think firstly, like with most things, a few qualifications need to be made, (1) That economic and monetary situations were highly variable both geographically and temporally, (2) that in Northern Italy, specially closer to the German-speaking lands to the north, and "Slavic" to the east, there was more than one coinage system present, and that the rates between them were highly contentious, and a subject to plenty of disputes, though perhaps we can avoid this, if we limit the time-scale, but is nevertheless an important thing to keep in mind if we look at the broader picture, (3) *Byzantine influence up to the 8th century was still considerable, probably as much as the Lombard in Northern Italy.

*(3) Franks reformed the monetary systems and introduced predominantly monometallic system based on silver in the 8th century, and the bimetallic system reemerged later in the 13th century, mostly due to the northern Italian cities, like Florence, where it was called Florin.

Now, if we settle some of the ratios for the late Roman Empire;
- solidus ( 4,5g or 1/72lb)
- semissis ( 2,25g - half-solidus, though this type was rather rare )
- tremissis ( 1,5g - worth 1/3 of a solidus )

Silver and bronze coinage;
- siliqua ( a silver coin, cca 2g, but ranging to as low as slightly above 1g )
- nummus ( by the 5th century the only bronze coin still minted officially, cca 1,4g and dropping to cca 1g - called minimi )

But this system went through significant fragmentation, starting in 5th century onwards, and even in the Carolingian period, some places with stronger Byzantine infuence that were Carolingian merely nominally still used gold based coinage. Ostrogothic coinage system in 5th and 6th century, while definitely more simplistic than its predecessor, imitated Byzantine system.

The first significant deviation were the Lombard changes at the very end of 7th century under Cunincpert, both iconographic and a value change to tremissis to essentially match them to their Byzantine counterparts, and the following change to sliver-based system, alreadly starting in France during the Merovingian period, but expanding elsewhere under during the Carolingian expansions. ( Though the exact development of the Merovingian silver system and respective failure of gold standard there is a contentious issue ).

On a careful note, one shold not mistake the early medieval gold solidus from above to the later silver Carolingian solidus with a basic ratio of 1 pound - 20 solidi - 240 pennies, though this was likewise variable, specially in times of fiscal crisecs and conversions between diffrent systems, a well known one was around Venetian system and the bordering HRE, where it happened that different provinces ( Duchies ) at some moments had different conversion ratios - which was, incidentally, abused.

And with continuing reservations, solidus, probably even in this case already, served as a unit of account, and not an actual coin, amd the chances of this rising the further north one goes, with the diminishing circulation and the drop of quality of tremissi, and already in this period, the silver tremissi were much more abundant.