The Giants "trade chips" aren't actually very good ones because MadBum and Smith are half-year rentals. The market prices, as long as the team is really willing to deal them (sounds like may not be the case with Bumgarner) for these types of guys are not really high (see Familia and other deals from last year).
That's fair, although I wasn't really saying that Smith would cost great prospects -- just that the Giants probably wouldn't target a guy like Cave in return for him.
I'll note that Smith's salary is lower than Familia's, and he's also left-handed, which could help his market. Also MLBTR even noted that the prospect haul for Familia seemed light, and the Mets were between GMs at the time, so maybe it's not the best benchmark:
Despite the interesting upside for Wahl, the general consensus in the industry is that the prospect return in this trade isn’t particularly impressive, especially considering the trade value of Familia.
But yeah, pure rentals ultimately shouldn't cost too much. It might come down to something like what you propose (Yunior Severino) but I wouldn't expect the Giants to jump on that deal too early, and there's no guarantee that offer would win out either (other teams could offer similar prospects to Severino, and then it would be almost a coin flip to see which prospect the Giants preferred).