Right now, according to CapFriendly, the Blues have just under $17M in cap space for next season with 8 roster spots to fill. Adding the pretty safe bets of...
Sunny
Blais
Dvorsky/Snuggerud/Bolduc
Kessel
Perunovich/acquired 6/7 D
... you're adding no more than $5M to the cap for those five slots, leaving you with 3 to fill. I expect you will see at least one other forward slot filled with at $1M or less, leaving you $11M to go after one big acquisition, presumably a LD. Assuming no roster subtractions to acquire that LD, that would leave a roster something like:
Buch-Thomas-Kyrou
Neighbours-Schenn-(Snuggy/Bolduc/Dvo)
Saad-Hayes-Torpo
Blais-Sunny-Walker/Alexandrov/New Guy
Leddy-Parayko (1A pairing)
New LD-Faulk (1B pairing)
Krug-Kessel/Tucker/New Guy
Binnington-Hofer
All of this is to make the point that, by trade or by signing, we can easily afford to add a $9M talent to plug the need for a top pairing quality LD next season and have plenty of cap space with a 22 or 23 man roster, depending on whether or not we spend some money on a pricier forward for the 3rd line and move Torpo back down to the 4th line.