This seems to be a commonly held worry, and sadly it might be holding people back from making Kitely a home base.
Those of us who have held events on Premium Worlds open to the hypergrid or whose Premium Worlds have featured in hypergrid safaris know what those costs are in reality. So this thread is to look at those actual costs, in the hope that it will help to allay fears and encourage others to take the plunge!
First, remember that other Premium Account holders do not cost the Premium World manager anything when they visit. According to the Kitely Services page under "Which Option is Right For You?" the Premium Account is the most popular option:
Buying Kitely Credits at the maximum discount gets 300 KC for 1 US dollar.Most of our paying customers opt for a Premium Account. The second most popular choice is a fixed-price Starter World. Many users also choose to combine these options, having both a Premium Account and a Starter World.
Costs of keeping Premium Worlds open for Regular/Hypergrid exploration
The particular focus of my own Premium Worlds is to encourage exploration. They are part of the Devokan group of storybuild worlds, so get a fair bit of exposure. On average, single explorers (Premium, Regular and HG) spend about 15 minutes inworld, with some repeated visits. I've been in Kitely for about two years (with various avatars). The highest monthly total came to about 10 Regular/HG visitor hours, or about 600 KC, or just 2 US dollars total for the month.

Since I have a Premium (Silver) Account (the grandfathered variety with 10 Premium Regions), this means I can sponsor a single Premium World (Ener's Hax Nuit) with 37,000+ prims, and develop a 3x3 Advanced Megaregion Premium World (Evensong, which melds the old single regions of Retreat, Skysong, Touchstone, Deepwater, Waysmeet, Tao Lia/Rivages) for the terribly affordable total cost of around 22 dollars a month for two worlds with far greater capacities and capabilities than available from other grids or vendors at a similar price point, and enjoy the benefit of all Kitely's enhancements over vanilla OpenSim.
Costs of well-attended one-off events
What about well-attended one-off events, such as the recent extended visit of the Hypergrid Safari to Imladris? Paislee has checked her account history for this event: 22 avatars attended, including Premium, Regular and HG visitors; each were inworld around 30-45 minutes. Yikes! That sounds like it is going to be expensive! And yet the actual total cost to Paislee for this event was 535 KC, around 1.79 US dollars for this one-off event.

Costs of weekly events regularly attended by a handful of people
Here I have in mind a group of four friends who meet up each week to play a game of cards. The game lasts 90 to 120 minutes, so would cost the Premium World manager between 360 and 480 KC per week, say on average 420 KC x 4 weeks = 1680 KC per month. That is 5.60 US dollars per month.

Now compare that with the current (actual) real cost of 9 US dollars per month to rent a titchy 450 prim plot specially for our card games, on a grandfathered full region in SL where the sim owner passes on the tier discount to renters. As soon as we get a version of the card game working in OpenSim (and it's getting close, a matter of weeks), we'll be moving the regular meeting over to Kitely. Then, bye bye SL!