They are slightly cheaper, but the prize difference is rather small- within the 100-200 gold range. Which, given how much gold you can get from slaying an enemy in battle, the number of tries required to beat the battle, the number of turns and general time taken to beat the battle to gain money, might not, in the end, be profitable at all. For instance, I can save up quite a bit of gold by bying only Swordsmen, but then I'll have a much tougher time, even if I invest the leftover gold, beating higher league battles than if I invetsed all that gold in, say, Spriggats, who have several advantages, including higher mobility and elemental resistances.
Though, for "proper" comparison one would have to start with 2 teams, one filled with Spriggats, one with Shadowlings, take the difference in gold required to buy the two teams and invest it into the cheaper team. Then, see what stats the team produces in terms of attack, health and defense (the way to invest would be to try and level up the differences in the attack stats, such that when the team goes onto the battlefield, the attack stats are more similar to each other). Then one can analyse the difference in stats at same Psy attack and defense levels, see which unit is more effective, then see how much gold is required to get the weaker teammate to the same level as the other one. Then compare all the invariable stats, then try to compare all the variable stats in terms of either gold invested or unit stats with same basic levels. Then try out a couple of battles of different difficulties to see which team is, overall, more efficient. (For instance, taking on Dark Spriggats with Spriggats has advantage of dealing more damage with Heat and Cold attacks, but the disadvatnage is that the teammates will be slowed down and will recieve the full blast from Dark Breath, unlike Shadowlings, who can get Shadow resistance, but would deal only half the damage to Dark Spriggats. In this situation, I'd probably say stick to Shadowlings, because, at least, your teammates will not be slowed down, whilst them taking to kill a Dark Spriggat twice as long will be balanced by them recieving only half the damage. )