replacing a 355 soft top?

That depends on your skill level. Here is a pictorial of replacing an old Detroit muscle car's convertible top: http://www.moparmusclemagazine.com/howto/155_0310_replace_convertible_top/

and here are the instructions for replacing a Mercedes SL convertible top: http://www.worlduph.com/mbtopinstall.htm

A 348 top is perhaps somewhere in between the two, though I haven't done it. A 355 top is powered, so it will be harder than replacing the 348 top.

The whole Ferrari 348 OEM convertible top (supports, shocks, hinges, etc.) is $16,000.

The cloth cover, if purchased aftermarket, is only $900 to $950 if you know what you are doing. http://www.the348.com/tech/348.html#ConvertibleTop

So if you know about the 348 Primer, then you can replace your 348 top yourself for less than a grand. If you just take it to a Ferrari dealership and opt for a full replacement, then parts alone are more than $16,000: http://www.eurospares.co.uk/partsListing.asp?M=1&Mo=559&A=2&B=30695&S=&ID4=1264934

You're looking at $25,000 for the whole 355 top ( http://www.eurospares.co.uk/partsListing.asp?M=1&Mo=522&A=2&B=29227&S=&ID4=1204023) if you go OEM!